I think you mean resistors, not registers.
Your right about the batteries. I forgot it took six AA. So three lithium ion batteries wold be 11.1v. Finding the right size is going to be tough.
Resistors would have to be replaced so that the capacitors and strobes are protected. Increasing the resistance right behind the strobes (my flash has two) would decrease the voltage back down to a safe level.
It's only a 2v increase so I'm not too worried about the capacitors.
Here's a similar project. Someone made an external battery pack for their metz. 7.2v