I have done some FBX importing today with Unity.
When I checked the contents on disk I saw it keeps the original .fbx in the folder structure along with plenty of Unity-only meta data.
So you may have a chance to grab the FBX (if original format was fbx) from the Unity project folder. And then you have a chance to go on with Blender.
Also get Makewalk (it's free and a part of Makehuman - http://www.makehumancommunity.org/wiki/Documentation:MakeWalk). It provides some Python Scripts for Blender for pose mapping.
I tried that in the past and mapping was possible, although sometimes somewhat fiddley, since I'm not very familiar with Blender.

...and if you already have contacted the dev... why not ask for a special deal leaving the unity package, the asset store and any possible legal issue with it aside and get the original model directly?