We used a second 12v-5v power converter (
http://www.andymark.com/product-p/am-0899.htm). It was a problematic because if the compressor and shooter motors were both running at the same time, there wouldn't be enough power left for anything else.
I realize now that was because we chose our gear ratios terribly and the shooter motors were running very inefficiently, but all the same I wanted to try to isolate the RPi's power from the rest of the robot so it wouldn't cut out.