You have an IGT Player's Edge Plus (PE+) video poker machine.
Your machine does support bill validators, so yes, in theory you could add one. However, on the slant top machines, the bill validator was designed to go in the top box of the machine, which yours is missing:
For home use, you could possibly get away with adding a side-mount bill validator box to the side of the machine, and running the wires through the base of the machine over to the motherboard. You would need one of the assemblies as well as the wiring harness to make that work. Probably hard to find in 2016, but they're out there. I'd start by contacting the vendors on the front page of the site to see if they have any. You're looking for a side-mount bill validator box with wiring harness for an IGT S+ or PE+ (the same unit was used on both the S+ slots and PE+ video machines).
(Photo credit: FORDBS)
Also of note, the validators for the S+/PE+ only go so far in accepting newer bills. None of them will accept the new purple-numbered $5 bills, or the latest $100 bills, nor will there ever be software to upgrade them.
Good luck!