I ran into this exact same issue, trying to convert floorplanner.com FML to a 3D model.

I decided the easiest approach was to use blender to programatically import the FML file, including all the textures and 3D models.

I created a python package that can do this and decided to open source it:
https://github.com/daviddemeij/fml2blender

Hopefully this will be helpful for others as well