(Partway down the page: FBX Converter Transfer project data from one application to another quickly and easily with the FBX Converter. This utility lets you convert OBJ, DXF™, and 3DS files to and from the FBX format.
I found this when trying to find a way to take an FBX file with animations and import it into Blender.
I’d try Carrara or Ultimate Unwrap 3D. Carrara opens .fbx files and can save to a number of different formats. UU3D imports and exports almost every format you can think of. Getting the animation into Blender won’t be easy though. Last time I tried, I couldn’t find a format that I could import into Blender and keep the animation. But that was several years ago, haven’t tried since.
BK, that’s the wall I seem to be running into… And I can’t seem to find the chink in the armor yet. I was hoping that Autodesk itself would provide the tool… but no dice. Not sure if it’s these files, as they are ONLY joints and animations, and no real model to speak of. My next shot it to try to assign this motion data to an actual model, and then try to convert.
When I was trying to do this, some while back, the only way I was able to import the animation was via a .BVH file. Things might have changed since then so you might want to try the blenderartists forums for more help. Otherwise, I found Carrara to be a pretty easy to use/learn tool and use it in addition to Blender all the time. Also, if I recall correctly, UU3D also allows you to merge mesh + animation (haven’t tried it in a while) so that might be worth looking at also.