Unless you have magic on your side, Nex and iPhone are two things that will never cross paths.
As I assume you know, DarkPlaces, the engine Nex runs on, is a very high end, high resource gaming engine that has no issues bringing high end machines to its knees.
But even if you could manage to ad an external GFX card and compile DP to run on the Arm, controls would be an issue as FPS are designed to be played with a mouse.
Be my guest and give it a shot though...Id honestly love for you to succeed.