Overlay/blend more than one animation

Hey,
I want to apply more than one or two anims/bone-positions to one skeletal asset.

Because:
I am messing around with morph targets (whole character body), for example morphing from 1-year-old baby to 20-year-old guy. My problem is, using bones alone won’t work (had to place dozens of extra bones). Morphing alone doesn’t work as well, cause skeleton doesn’t fit anymore. Scaling isn’t working, cause leg-arm-ratio etc.pp. isn’t constant.
So I rigged the baby, rigged the guy. Afterwards I applied the guy-skeleton to the baby-mesh, to get something like an deformed offset-mesh. Used this mesh as a morphtarget.
Now I can morph the guy to the deformed baby-mesh and blend the baby-skeleton in as an animation. Works so far, but - how do assign another animation, like walking around? Also more morphtargets should be included, therefore I have to blend even more skeletons/animations in. For example small/tall, skinny/fat,…

Or is there another, better, workflow available? Additive blending, blend spaces, manual bone-offsets?

sorry for my poor English,

greetings
David