If you wanted to get real fancy you could figure out a way to run HPA through the pistol grip and somehow put the feed lip from a gbb magazine on a normal highcap. But, if your going to do this you might as well buy an AEG.
Contrary to what one poster stated, Polar Stars were not created to cater to the GBB audience who also want to use AEG mags. Polar Stars were created to cater to the audience that wants to be able to easily tune their existing rifles, and wants better performance than a gear box can offer, regardless of added price. There is, in fact, no blowback mechanism on Polar Stars.
However, Daytona Guns/TNK Guns WERE created to cater to the mentioned audience. as CoppertopNeasg stated, the ideal solution for you would be to run a HPA line through your pistol grip, and use AEG mags. TNK/Daytona gun (henceforth referred to as DG) systems completely replace the internals of your existing AEG or GBBR body, depending on what DG system you buy. They do not have a huge lineup of rifles listed on their website,
http://www.tnkguns.com/Airsoft.html but they have offered and/or continue to offer many more platforms, such as the SCAR Mk16, the L85, the M60, the FNAC, the ACR, and more.
DG/TNK system rifles are extremely reliable, have the most realistic kick on the market, and have very high quality internals. DG/TNK system rifles can be had at or around the same price as Polar Stars, and are arguably more reliable, depending on who you ask.
You can check out the DG/TNK system (demonstrated in this video video by the TNK SCAR Mk16) on one of their hand full of YouTube pages here:
Again, to clarify, the TNK/DG system is a gas blowback system that uses an external HPA rig, run through either the lower receiver or pistol grip, depending on the rifle platform and user preference, and DOES use AEG compatible mags.
Hope this helps anyone who is interested in the OP's concept.
Have a great day!