There are premade packs for the 3PK all over the web, made with various NiMH cells, like there are a few on ebay right now made with 2700mah Sanyo nimh, premade 8 cell flat packs ready to go with connector, or you can get crafty and make your own pack.

Before you get too concerned about not having enough capacity with the 700mah pack, I'd say try it first.