No you can only use either the canon batteries (one or two at a time) or the AA batteries (the case houses 6 and as far as I know needs all 6 to function). There are infact two different trays for the grip - one for the canon batteries and one for the AA.
Personally I have four canon batteries, but with two in the grip it lasts for ages, you can cut down to one at a time if you prefer (that has the bonus that you get time to charge one up, instead of having both die at the same time and thus leave you with nothing to use).
Tools of the trade: Canon 400D, Canon 7D, Canon 70-200mm f2.8 IS L M2, Sigma 120-300mm f2.8 OS, Canon MPE 65mm f2.8 macro, Sigma 150mm f2.8 macro, Tamron 24-70mm f2.4, Sigma 70mm f2.8 macro, Sigma 8-16mm f4.5-5.6, Raynox DCR 250, loads of teleconverters and a flashy thingy too
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