Not directly. You can basically use these 2 ways:
- add your new commands directly to the installed/Mod/Part folder, this is more if you are thinking of distributing your code or submit it for inclusion to the freecad code
- add your command to a separate module (another folder in the Mod folder), but don't create a workbench, only the command. Then, in freecad, you can add a custom toolbar to your Part WB with edit->Customize, then add your new command to it.