Add an event after the add waypoint event -
+ System: While
+ Waypoint: Waypoint overlaps Wall
-> Waypoint: Set X to Random(DisplayWidth)
-> Waypoint: Set Y to Random(DisplayHeight)
So it will keep randomising the position until it no longer overlaps with a wall.
Totally untested, but I think it should work. I've used a similar technique in other software, in the past.
EDIT: Just looked a little deeper and realised your waypoint is not actually an object, but is an option on the RTS behaviour (which I've never used), so my solution above won't work. However, I'm confident the concept will work with a little tweaking - just use a 'while' to relocate the waypoint until a usable position is found. I'll see if I can come up with a proper solution.