A 1" x 8tpi tap that will thread steel is likely to be very expensive. (I have a tap to thread wood, but there's no way it would thread metal.)
I think your best bet would be to find a nearby metalworking company, or an old fashioned hardware store, and ask if they can help you (1) determine the thread specifications of the chuck, (2) figure out if the chuck has an insert to adapt the threads (such as the Nova chucks, for which you can get inserts to make the chuck fit lathe spindles with 1"x8, 1.25"x8, M33, and so on).
If the chuck doesn't have an insert that can be removed and replaced with a 1"x8tpi adapter, I'd ask if they could re-thread the chuck for you. It'd be a lot higher chance of running true if the threads were cut by a professional than by me.