[Imp]

This could be wrong but
The foxhole is part of the unit & is placed in its start hex with it.
Thats why when you see a unit that has left its foxhole you see its foxhole to.
Using enemy foxholes trenches is quite possible, takes a turn to get the dug in status.
Very useful if defender has placed close to objectives as enter & throw smoke, when it clears welcome to WW1 as 2 dug in armies now duke it out. No idea if the morale boost I think you get for being in near a fortification applies if it belongs to the other side though.

Why not just place trenches if you do not want units to start in those positions? Also gives benefit of being able to move in them & retain protection. Drawback is they are visible from the off I suppose.
Would suggest buying guns first & having them dug in, as its a base the troops might still be in the barracks but the guns would most likely be emplaced.