Well, you can open up one of the four official "special grahpics" levels in LemEdit. LemEdit won't be able to display the terrain because that comes from the extended graphics; however, I believe it will still let you edit certain level data such as the title, skillset, release rate, etc. Maybe even editing interactive objects and steel areas (but it'd be kinda hard doing that without being able to see the terrain). Then save the level into your own .dat file.
From what I've seen, it seems that when you use an extended graphics set, all the terrain layout info is taken from the vgaspecX.dat file rather than in the level data, so there's pretty much no way for you to change the terrain without editing or creating your own vgaspecX.dat file. At this point you're on your own.