I think I tried that with a couple of levels once. I also once took the levels from ONML and ran them with original Lemmings, which is quite easy to do on the Mac because you just move a file over. Basically, you get lots and lots of jumbled-up terrain. It crashed a lot, though, because of any invalid tiles that happened to come up (like the brick tileset from ONML has a lot more valid tiles than the rock tileset from L1), so I had to keep going through and changing the tileset until it worked.