Thanks for your comment, scdbs.
Yes, I used this trick with Superior Drummer 2. However, SD3 now supports a new "User Libraries | User MIDI" in the Grooves panel with more and new functionalty:
1. SD3 contains a new "Add subfolders" button on the User MIDI to create new directories inside SD3. That's not available on "Factory" Midi directories.
2. SD3 contains a new "Sync File Changes" button on the User MIDI to synchronize file changes on disk. That's also not available on Midi directories.
3. Manually modifying the Midi directory requires a restart of SD3. That's not needed with the "User Midi" which can be used inside SD3.
4. The "restore MIDI database" is an emergency button, because it erases all favorites and rebuilds the MIDI database.
I like the new "User MIDI" directory and is a great improvement compared with SD2. So my feature request is to put this folder on top of the list. 