I have a 2 1/4 by 3 1/4 Century with 2 120 roll backs. I just purchased a 4x5 Speed with a Graflock back. My question is, can I use the roll backs on the 4x5 and how do I convert them so I can use them on both cameras?

Nope - and unless you have a machine shop, you can't.

There are special models of the backs for 4x5. No "reducing" Graflok back for the 4x5 that I know of.

Now, if you were to use the mating part of a 45 film pack adapter, with a 23 Graflok back, .......