Dell didn't seem to design the Mini9 with any intention of ever adding more cells to the battery. Can't go out the back with more cells, you wouldn't be able to open the screen, can't go up, you couldn't shut the screen. Can't go down, it wouldn't sit flat on the floor, and people already complain the screen doesn't tilt back far enough to see properly, so if the bigger battery is forcing the unit to lean forward...
I have seen eBay sales for external battery packs that will give the unit a recharge when it gets low.
Or like mentioned, buy a couple spare battery packs from Dell to swap out as needed. - Charging all of them can be a pain, I imagine.