You can always use Autodesk Maya with FBX plug-in installed so you can export .mb files as .fbx files directly in Maya. If you do not have the access to the original software, you will have to rely on some 3rd party CAD software or converter, for example the PolyTrans|CAD+DCC.
Convert Autodesk Maya binary scene to Autodesk Kaydara 3D data exchange format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert mb to fbx on Windows
A 3D CAD translation softwate
Convert from mb file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.