Well, when there are no more enemies detected in the vicinity of the convoy I assign them as cargo to their original ural and then use the orderGetIn true command for each of them.
It looks good, they get out, deal with the enemy and then get back in and the convoy continues on its way (I am still only testing with enemies that don't shoot back though!)
If their truck is dead then they will just hang about at a Guard waypoint at the location of the last detected enemy. That would would be quite good enough for what I want.