Is it possible to import or edit material into Sketchup Shop?
Yes, you can, see how:
Don, here’s an alternative to the method shown by mihai. If you know at least one dimension of the material, import it as a material and apply it to a face with the known dimension. For example, in the GIF below I know that my wood grain material is 8 feet long so I drew a rectangle that is 8 feet long to put the material on. The width of the rectangle isn’t important because the texture width takes care of itself.
Thanks! Is there a source for those image files?
I make my own wood grain images. You can do that, too, with photographs and an image editor.
No way to use SKM files?
Not in Shop.