An example is always best for figuring these out. There's a Fighter's guild member in Chorrol who does exactly that, Kurz gro-Baroth. Check out his setup.
I tried that, but I didn't really understand anything in his setup, rendering it useless for me...
Some other quick questions:
What does "Value" under the AI package menu stand for?
And does the NPC use waypoints to cross distances between x-markers?
And how do I change it's AI package to let it spawn when the PC leaves the area?
What does the regular x-marker do (not x-markerheading)? Is that just a spot for a NPC to go without standing in a certain direction?