Make the wall Walkable (can be done in the Object Editor). However, you'll have to place Pathing blockers (Ground) on the edge of the wall, so they dont walk off the edge. Like this:
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ <--PATHING BLOCKERS (GROUND)
==================== <--WALL EDGE
--(Units can walk here)--
==================== <--WALL EDGE
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ <--PATHING BLOCKERS (GROUND)