I found these rosters for the Franco-Prussian War that can be used with Black Powder:
French
Prussian
For 6mm Black Powder, I'd recommend changing all of the measurements from inches to centimeters. This will decrease ranges and movements by 2/3, giving you a larger scope of engagement for bigger battles.
Another option would
be these house rules.
Getting Black Powder to work for FPW may be difficult, since it's far along enough into the "Rifle & Saber" period that Black Powder stops being useful as a ruleset.