I believe you can. It might make it a lot heavier though, just make sure it is non-toxic.
Make sure you definately seal it off when project with acrylic sealer.
I'd be wary if your going to use cement.
It will be pretty heavy, and if your going to make ledges, or raised platforms, steps etc, then be careful that the joins you make are secure when fitting the ledges ... the extra weight might put a strain on the foam ... and the last thing you want is a ledge to break off and possibly injure a gecko.
Do you know a other name for the mud that you use maybe? We have 2 big store that sells all kinde of stuff. But i dont know what i can call it in dutch.
How is the drywall mud holding up? Any chipping, breaking , cracking or anything? I'm making me one and using it also instead of grout. Pretty easy to work with. Just wanting to know its durability so far, being you have had yours about a month or so in use. Thanks!