The original files inside the .dat files seem to be in another format and other file names.
Level images are .PCX
Sprites are .GOB
Sound effects are .SMP
And music is .MOD
See here for description of the files: http://www.aiei.ch/linux/jumpnbump/level-making/
If I want to get the levels working I guess I need to convert the level images and rename those files to match the filenames currently in the custom folder. Does the game currently support the custom sound effects, sprites and music?