Hitec sells a battery tray specifically for using alkalines. You can't use them with the stock setup. Several guys in my area run the 3D; some have the alkaline tray as back up, some have a spare nicad pack, and some don't have anything for backup.
Personally, I upgraded the pack to a 1600 mah nimh pack from
www.maxxprod.com . Also bought one of their overnight wall chargers specifically designed for nimh packs that will charge both my tx & rx at the same time (just like the stock charger that comes with the 3D, but designed for nimh packs). I can run 3 classes all weekend without coming close to killing the tx pack, and the 1100 nimh Orion packs give me PLENTY of run time in the vehicles.