Not quite sure I understand the question, but on the Genos I have several folders with registrations. When I want to create a playlist I navigate to Folder A, take the registrations for the songs I want to add to the playlist. Then I navigate to Folder B take the songs I want to add to that playlist. From there just repeat the above until the playlist is complete. As far as I know you cannot add all the folders at the same time but not sure why you'd need to do that anyway? At least the way I work with folders/registrations.

Fran is on the right path with his explanation.