You can convert .fbx files created in MotionBuilder2012 to an earlier version of .fbx file format by choosing from the main MotionBuilder menu Python Tools > FBX Export > FBX 2010.
Python Tools menu
Saving MotionBuilder2012 to FBX version 2010 ensures compatibility with Autodesk version 2010 applications.
FBX Export
FBX files exported via Python Tools > FBX Export > FBX 2010 tool or the FBX Converter tool are FBX SDK files and not MotionBuilder files. This means that anything specific to MotionBuilder will be discarded (such as story information, Devices, etc.).