The program only accepts .vox. Formats like obj and stl could be any mesh, so there's no way to guarantee that they represent voxel models, or conveniently shaped tiles of any kind.
The program doesn't export maps. The idea is that Generate Worlds is that walking around experience. Because it's not a static exported map, you get to seamlessly explore an infinite world that's built around you.
If you're inclined to write software for VR or browser, the algorithm that powers Generate Worlds is public. I've written it up here, so you could use that to implement it for any platform. If you have questions about the algorithm, shoot me an email!