From Wikipedia: OpenArena is a free and open source first-person shooter computer game based heavily on the Quake III Arena-style deathmatch.
So this has nothing to do with emulators. However, if you are playing RetroPie with your friends and want to show them what the Raspberry Pie is capable of, you can start Open Arena right from emulationstation. It has great graphics and runs full HD. However, it is nothing for kids.
That said, here are some easy steps to get it into RetroPie.