Well, I'm no doubt sure there are probably plenty of answers to your question, it turns out Yoga itself can be a way to stretch our emotions. Many yoga instructors will remind us in class that a lot of poses, especially ones which open up the hip, can often be emotionally strenuous as well as physically. It's not really something I can begin to explain but it is something I have felt. There have been times during the tougher hip poses that I've felt tears spring to my throat and it's not uncommon for people to cry in class. My one friend was in a class once where the woman next to him began to sob uncontrollably during the half pigeon which is a huge hip opener. While this response isn't exactly common I think there is definitely a connection between mind, body, and emotion that can all be bridged in a smart yoga practice. 

You've asked a definitely solid question. It's hard because the other two examples of stretch are so much more tangible. We know exactly what's good for our body and I think most of us pretty well much know what food is good for our minds. But when it comes to emotions, which are so much more spontaneous and improbable and mysterious, how do we know the proper ways to treat them? Well, there are lessons from the other types of practice that we might apply to emotion. 

How can one practice feeling? Well, emotions as far as I can tell, require at least two parties.  Our feelings origin from relationships. This can be a relationship with ourselves, with a friend, lover, parent, pet, memory, idea, and really practically anything. It's the brightness of our species that we have this uncanny ability to create meaningful relationships with absolutely anything. And as these relationships evolve so do the feelings created. The longer the relationship goes on the more wild and complicated the emotions get. 

I think a large part of your question has to do with keeping these emotions both meaningful and manageable. How many of us have been remarkably discouraged when our feelings for someone or something have gotten so mixed and muddled because of all these different reasons that the only thing that makes sense to do is to leave it all behind.  If only we had stretched our emotions and feelings before things became irresolvable. And for this we need that emotional stretching. What that entails? Truth. Be passionately honest to all these different relationships we have. Let your mind and body inform your emotions because they often know what we are feeling before we do. And to this I think yoga itself is instrumental in the calming of our minds and bodies, a calming that engages us with our most overlooked sense, emotion.  

emotional stretching, or the lack thereof, is actually why my last relationship had to end.  i'm a very emotional person.  i feel intensely, the highest highs, the lowest lows: i qualify as a Highly Sensitive Person (See the work of Elaine Aron, PhD).  My former partner however, had a much more limited range of emotions.  they were basically mid-range, never to high or low; he had emotions, he just never experienced them intensely.  In hindsight i understand this, however when i was in the relationship, i was frustrated bec i felt my emotional needs were unmet. 

we actually saw a psychologist together, and  realized that what we were really missing was empathy.  bec of the difference in the range our emotional experiences, we effectively could not understand each other.  and no matter what we tried, it was impossible for him to "stretch" his emotions to meet mine, or for me to delimit mine to meet his. 

so given my personal experience, i don't think it's possible to "stretch" our emotions beyond that which is natural to us.  however, i do think its possible to stretch our empathy and understanding of others; we just won't "feel" it the way they do.     

Penelope Rose.
I regard your idle question to have meaningful consequences depending upon the responses your question elicits.

It seems to me that emotional, conscious stretching happens when we "walk in another's mocassins (to use the time "worn" shiboleth) We truly can practice emotional yoga (and many do) when we endeavor to experience what the emotional experiences are or have been of others than our selves; it is a "stretch" to practice feeling the others' feelings that drove their actions. For the sake of shock I offer the following example from within my family's history.

When my father's father was dying of heart failure after having survived a stroke, he lay in bed rambling about his regrets. I was two years old and so my understanding of this is second hand. When he travelled from Ohio to California in 1890 by horse back with his brother they encountered Indian women, alone along a river, washing clothes. As they rode up to these women, the Indians ran to the river bank, squatted and filled their vaginas with sand. In my grandfather's ramblings he confessed responsibility for their feelings. 

I don't know if he had intended to rape them or not, but certainly the notion was within his remembered grasp. He expressed great sorrow during the memory. I think that he was practicing something akin to emotional yoga. I don't know if he did at the time that he and his brother encountered them along the river or only in restrospect on his death bed on the ranch, or both. My sense is that he felt the emotional feelings that the women had felt towards the implication of his potential threat to them and he was experiencing their emotions in his memory as his own, while being them, looking at him both historically and in his present postion of memory.

When I watch my grandaughter exploring her world as it expands within her search for control, I feel an out of body hum of being her, of myself being her, feeling her flights of frustration and joy as she plays, realizing just how intensly hungry she feels when she feels hungry, (yet  I am not hungry), feeling the profundity of her anger when she doesn't get her way, her bright eyed, delight in doing somersaults as she tumbles about the pillows on the floor, awash in the joy of her body, while I sit arthritically on the couch watching her antics. 

Perhaps these are snap shots of a kind of emotional stretching? Whatever they are they are very necessary to our healing of feeling alone.