Look at this:
http://www.ryandor.com/forum/viewtopic. ... e+mountain
Technically, Syd is right, however, Dragon is designed so that you can draw the cave floor, and DragonSP will place the static cave floor tiles. You then go into WorldForge and draw over the area with rock, effectively replacing the cave floor map tiles with the mountain you need above. You'll end up with a statics only cave floor.
Also, make sure you place a 2 pixel or more (3 or 4) black border around it as well so that the cave walls get generated.
Finally, you'll need to manaully add the static cave floor through the entrance.
-Ryandor