In order to open and edit fbx files we recommend to use the following tools: Paint 3D, Blender, Autodesk Motionbuilder and Mudbox.

2] Blender Blender is among the most well-known open-source 2D and 3D software tools. This program is another excellent choice for Windows users who want to open FBX files and other 3D file ...

If the .fbx has the same bone names as the MMD armature, you can open a graph editor viewport and copy/paste keyframes from one rig to the other rig. Go to frame 1; select all bones in pose on .fbx, ...