Apologies if this was answered but to the original question, here’s a simple breakdown. Hope that helps.
Step 1: Edit your sound (pre-made or from scratch)
Step 2: FUNC + IMP/EXP
Step 3: Select: Export Sound
Step 4: Select Save Location (Starts you off at C:001 since the previous 256 come with. Level Data knob to scroll up)
Step 5: Name your SOUND
Step 6: Select which categories it applies to. (Easier to find via category search later)
Step 7: Hit ‘Save’ (first option on that category list)
Step 8: Back on your project, FUNC+SOUND BROWSER will access all of the sounds. Yours included. Tab left on that screen for filters.
Then:
FUNC+IMP/EXP
Manage Sounds
Select Sounds
Tab Right > Copy To:
Sound Pool