FBXLoadImportPresetFile -f [filepath];

This command lets you set the import preset file path.

You can use this to load the MotionBuilder Import presets without knowing their file path.

For example: FBXLoadMBImportPresetFile; loads "C:\maya2012\bin\plug-ins\FBX\Presets\2012.0\import\Autodesk MotionBuilder.fbximportpreset" (The path may be different on your machine.)