There is no probability of presence option in the Elite editor.
The only way to do it would be to make the mission on the pc and then port it across to the xbox.
It sounds much harder than it actually is
gabrieladepaula did a good porting tutorial find it here - link copied from porting thread
Keep in mind the poxy amount of RAM the xbox has so don't get carried away placing units/objects- did anyone upgrade the RAM on their xbox?
Cutscenes have to be brief. The camera commands act differently so don't do anything too fancy.
Everything else is fair game ***
Most missions will work yes. I even got a couple of mfCTI missions working ok-ish. Had to scale them down a little.
Red Hammer was working well, except I couldn't get it to autorun from the campaign screen.
A unit on a "Guard" wp will hold that position. They will respond to reports of enemies elsewhere on the map.
For example, you had a Cobra on a "Guard" wp and a unit reports "T72 at 12 o'clock" the Cobra would head towards that direction, take out the tank and then return to it's previous position.
If several units are on "Guard" then only the most suitable unit will respond depending on the type/strength/distance to said enemy unit
Sometimes if you annihilate a unit really quickly they won't have got off a report hence no guarding unit will come so yes, I suppose that does add a randomizer to the mission ***
I like the ingenious methods Elite players came up with to get around the limitations of the editor. I used to use the "trigger trick" all the time.