You can add filler to powder coating. It is called Lab Metal. It goes on like bondo I guess. I just ordered my powder coating gun and a bunch of powder along with the Lab metal so I don't know exactly how it is or works. Here is the site for more info on it. You can sand it, drill it etc.. It is a high temp filler. I hope this helps! Click on Metal fillers on the left side of page.
http://www.columbiacoatings.com/800fsgfdsg45etresgv566765ytrh567.htm