Is there any software which would let me import the existing models, define the bone hierarchy and the model which corresponds to each bone, and export a single articulated model in T-pose?

I found that it is relatively easy to create articulations using Blender. This tutorial might be helpful  : https://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro/Bones

Is there any software which would let me import the existing models, define the bone hierarchy and the model which corresponds to each bone, and export a single articulated model in T-pose?

I found that it is relatively easy to create articulations using Blender. This tutorial might be helpful: Blender 3D: Noob to Pro/Bones | Wikibooks