What is the point of creating a background in the main site if you can't even use them? There is, of course, a way to do it for real.
You can copy the information from those backgrounds and use them to create new backgrounds in the game. I have done it myself. Sometimes they work and sometimes they don't. It all depends on how well the background is designed and how well you understand making new backgrounds. THAT is the point of the Background Bank - I think.