Make a pack up from individual cells. (or have one made...)
The TX is FCC rated for having a given voltage range applied to the RF board. Changing that voltage could be a serious problem to the tuning of the radio. Don't mess with a low voltage pack.
It is PERFECTLY FINE to make your own battery though. I have an 8 cell pack (as per original) in my Futaba Super 7 made from Ray-O-Vac 1800 mah NiMh cells. I just about can't run out of TX power at the field.
Fairly simple to make the pack and cost me all of $18. (swiped a 3 wire connector with wire from an old computer... fits perfect in the Futaba plug and has heavier wire than the original. Radio South thought it was great.
(sent the TX in for a switch repair. Radio South does good work.
plug
plug
)