If you're asking how to, I'd recommend the second battery from Creative.
While NiMh cells are rated at 1.2v, they can creep up over 1.4 on a full charge so 4 cells can actually deliver too high a voltage for the JB3 - causing it to not operate. You may have to pre-drain slightly. Test it first - if it doesn't power up that's the problem. I ran a halogen bulb on it for a minute and all was good. BTW, 3 cells is not enough voltage (tried it).
Others (Dale I think) tried just adding a diode to a 6V (to cut the voltage a bit) is the huge intermittent power draw the JB3 needs. Tends to overload the diode(s).
Using regulators is great - but they generate heat and need ~1.7 v of overhead to operate. A 7805 regulator will do (cost <$1), but you'll need at least 6.7v source (i.e. a 6v SLA won't do it). Run a regulator straight off a 12v without a heatsink and you'll burn yourself if you touch it, and it will probably shut down after a few minutes anyways (built in thermal protection). With a 9.6v battery, it's not quite so dramatic. Haven't tried with 7.2 but that's where you probably want to be if you go this way.
The easiest DIY is to buy any old 'car adaptor', set it to 4.5v output and cut off the lighter adapter and power if from a 7.2, 9.6v NiMh or 12 v SLA. It has the regulator already heat sinked.
Also, anytime you hook up an external source, the JB3 will attempt to recharge the internal battery, so there's some strategy to the order that you use power. Because any interruptions in the voltage level of the external source will screw up your recordings, I'd recommend starting with external and yanking it at a known safe point, allowing the internal to take over. If you run out of juice then, the internal shuts down safely and saves your recording.
If you're looking for the JB3 connector, I had luck with a 1.75 X 4.0, wired centre positive. Even the Creative connector is kind of a sloppy fit due to the angle of the case.
So while I've had success with several methods, if not for my fondness for DIY'g, I'd just go for the official battery.
Understood - you have special rush needs so good luck!