Im in the process of changing the rear drums to disc and was wondering could I just use the the front brake assembly front a bonneville and use it for the rear?

in the process eh? whats your solution... nobody has never done it

no, just slapping the front brakes on will NOT work

Did they ever put 4 wheel discs on that body style of Bville or maybe a park avenue, maybe even a seville would work? That would be an easy swap? But I am no mechanic.

search for rear disc conversion and you will find overwhelming amounts of information

This conversion may be more trouble than its worth. My GXP bonneville with 4 wheel discs did not see to stop any better than the 95SSE with rear drums I had and sold.

Take a trip to the junk yard and see if the rear kunckles and disks from a pre 97 w-body will fit. They have shocks in the rear like a pre 00 bonnie.
You'll still need to figure out how to get the parking brake to work and new proportioning valves.