I used to make radiation shielding for X-ray machines.
How large of a sheet do you need to make, and what form is your lead in now?
I can't see making a lead block into a large sheet using a slip-roll. You'd have to pre-flatten, and it would need to be done to a uniform thickness before you started rolling (Could be done in a large hydraulic press using flat dies).
A slip/pinch-roll machine, not being designed to make thick material thinner, may not be sturdy enough (due to the long length versus relatvely small diameter of the rolls) to do even soft lead over a large section of the rolls unless you're talking a large, power-driven machine suitable for forming thick plate.
A light-duty, hand-cranked sheet metal roller probably wouldn't work well; it will take more pressure than you might think. It would be very difficult to get a large sheet through and you'd have to put it through a whole bunch of times to get the thickness reduced much (if it worked at all).
That said, it might work okay to make a small strip as opposed to a large sheet. Would need to be done near the end support of the rollers on a sturdy enough machine. Need to use lube since it's gonna want to stick to the rolls, especially if they're dinged up or scratched. The more pure the lead, the worse it wants to stick.
McMaster-Carr sells small quantities of lead sheet but I don't know what they charge nowadays.