My $.02 here. A USB power bank is going to provide 5 volts. So a bit low for the 7 to 32 volt range spec'd in B2N2's manual. It may not work.
That said, it might. The slightly low voltage will probably not harm the headset. And the worst you will do is waste a bit of money on a power jack, USB cable and the time to cobble this together if it doesn't work.
Keep in mind that most USB power banks use Lithium Ion chemistry cells. NiCad (or NiMH AA cells) have a much lower propensity for spontaneously bursting into flames. So I'd tend to use the latter if given the option.