Adding ROMS difficulty?

I've recently started looking at buying a Legends Ultimate machine, as I was going to originally just build my own MAME machine from scratch. I have probably 100 or so original coin op ROMS for many of my favorite games. I ran across a Legends machine on FB marketplace for $400. It looks to be in very good condition and has the 5.70 firmware version and is model HA8801. How difficult is it to add my existing ROMS to this machine? I took a look at the list of games that are preinstalled and don't have much interest in them as most of these are console games.