I'd imagine he just soldered them together, much like how you do with the high performance cells that you use in 1/10th scale sedans and 1/12th scale cars. You just connect positives to negatives to series up the voltage, and at the ends you solder up a connector for the reciever.
Theoretically, you could go unlimited mAh if you line up cells in parallel and then string those sets in series. They have some 10000 mAh D-Cells you could get =) That is, if you could ever find a way to fit five D-Cells in your ride hah
I just use a venom hump pack 1100 mAh NiMH battery pack on my buggy. You can find these or ones like them at most hobby shops. The hump packs use small batteries so they are usually lower mAh - your Ofna and NTC3 might be able to use the generally higher capacity in-line packs.
But, with the 1100mAh I can use the thing all day without worries. Of course, that's at the track, where you run for 10 minutes then sit for an hour. But I've also actively run the buggy for hours and never had the pack die once. And I have some power hungry servos too - JR DS8611 for steering (Digital 260oz) and a digital HiTec HS-5925MG for throttle.
It also has a lot to do with the charger. I used a "quick 4 hour" charger at first and it just wouldn't charge the battery - and you'll kill it quick doing that all the time. Then I got this super-neato charger - the Double Vision Peak Detect Field Charger. You need a 12V power supply (cheap) or a car battery, and you can charge your receiver pack (4.8 or 6v), transmitter (JR and others - I have 8 NiMH AA's in my Xr3i and it charges them just great) and glowplug driver all at once. Does a complete charge in about 30 minutes from absolutely drained. It's not expensive, check it out! It's the ultimate charger for Nitro.