If I understand your plan correctly...you want to use the HOB filter by actually plugging it in to pump water into your sump and then have a powerhead(s) to pump the water back to the 30. Correct?
If so then I would advise against that. The flow of each of the two (HOB and PH) would have to match
exactly. Otherwise something's gonna overfill. Either the sump or the main tank. It's too risky, IMHO. You should have only one electrically powered resource in the loop (IMHO), which is most commonly a pump or PH in the sump since this is usually positioned below the main tank. The feed for the sump should not be powered, but rather, it should be a simple gravity feed by way of a main tank overflow, IMHO.
IMHO, get the tank drilled while you have the chance!
Cheers,