I haven't heard of a USB/firewire nubus card. I think you have to have a PCI slot for those. In fact, I'm almost CERTAIN that you have to have PCI.
That's like the rumoured PCMCIA USB card for the PowerBook 1400. They don't exist! There's no way to 1) go above the 1400's 64M RAM limit, or 2) add USB to it, regardless of what web rumour might say.
Good luck with your upgrade, though. I strongly believe in using a machine until it gives up the ghost. There must be SOMETHING an old 7100 is good for, even if it can't handle a USB card.