Assuming you had a "breathe" morph (more below) the coding is pretty simple and straightforward (this should give you all you need to get started):
import RLPy
avatar_list = RLPy.RScene.GetSelectedObjects()
for object in selected_list: #for all selected
if object == RLPy.EObjectType_Avatar: #if is avatar
avatar = object #just for reading purposes
morph_component = avatar.GetMorphComponent()
morph_names = morph_component.GetMorphNames("your_mesh_name")
result = morph_component.AddKey( "your_mesh_name", "your_morph_name", RLPy.RTime(67), 0.5, False, False )
The real problem is that morph -- can't program it automagically. So you would need to somehow develop it and, further, apply it (AFAIK you can't add a morph programmatically. I might be mistaken in this but it doesn't appear so from first glance, although I just woke up from a nap). If so, then the avatar you wanted to run this on would both have to have such a morph already applied AND the name would need to be standardized for automagic applying it (or you could have the interface select it I guess -- I'm not really going to do any UI in Python just yet but I know there are a few here who have who could help you).