I haven't tired it yet, but if you browse your materials list. You will see all of the materials have a .jpg extention. That suggests they are not a unique SU extention file type, but rather just a thumbnail of a jpg.

It seems you should be able to make your own lumber, or copy one. Then make it a jpg file and move it into your materials file.

Once you get it in there you can as with the rest of the materials manipulate it there.

For instance.....If you are using T&G flooring/ceiling. You can change the width of the boards change the direction the boards run to any angle. and a few other head scratchers I haven't figured out yet.