If you’re definitely intending this to be a 2D drawing (which apparently you are), it isn’t clear why you chose to use a 3D modeling program, which is really ill-equipped to do the things you describe. There are quite a number of 2D vector-based drawing programs, some inexpensive or free, that would probably do a better job with less effort. We could no doubt direct you to some of these programs if you wish.
On the other hand, since presumably the asbestos contamination actually exists in 3D (on walls, floors, and ceilings), perhaps you should consider using SU the way it was intended and make a 3D model of the structure, depicting the contamination in the actual locations where it is present. You could probably use a texture to represent areas of contamination. As far as that goes, that’s probably a good answer to your original question: you can create custom images with patterns of your choice and import them as textures.
-Gully