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:\maya2013\bin\plug-ins\FBX\Presets\2013.0\import\Autodesk MotionBuilder.fbximportpreset" (The path may be different on your machine.)