I don't have time to search, but would appreciate anyone finding/testing free/PD and alternate levels or even total conversions. Let us know if anything works.

Apparently you need to create a .duk textfile containing two lines

CON=.con file name
GRP=.grp file name

for instance for the Penthouse Paradise modification:
CON=PPAKGAME.con
GRP=PENTHOUS.grp

Put the .duk file and all the necessary data files in the share/duke3d directory and run the game.