I was wondering if a SM-BB7900 Italian thread bottom bracket would work with some FSA Omega two piece cranks. I know these are not classic or vintage but the bike they are going on is.

Google says maybe. Apparently, the Hollowtech II BB can interface with the MegaExo cranks.

Thare is a hat-shaped (i.e. flanged) sleeve spacer that's used for applications where one or both sides of the crank's spindle are smaller than the 24mm ID of a Shimano external bb.

If the FSA crank spindle is 24mm, then only spacers need to be considered, for positioning laterally and to eliminate any free-play.

Be sure to check that the chainline is where it should be, which is important wrt front derailer shifting.

It will work in the sense that you can assemble it and ride it. However, at our shop we don't mix and match FSA Mega Exo cranks/bb's with Shimano Hollowtech II cranks/bb's because the bearing preload is much greater when you install FSA cranks compared to Shimano. The manufacturers say don't mix and match these, and we feel it isn't worth any risk of premature bearing wear or other possible problems caused by the different designs. YMMV.