You can figure out how much malt extract would be required to get your OG where you need it after you max out your mashtun. I would think your only other option is two mashes, and then combining them.
And this isn't really a problem, as you can easily split the grains down the middle and have close enough to an even distribution to get really similar worts. It will just be a longer brewday. Personally, I would probably go the extract bump-up route. Unless you are that purist that has a moral objection to using DME in your AG brewing