Without spending a lot of time on this, it looks like the short answer is not easily or clean you can't.
Is your slot a straight line, or is it contoured?
It might be quicker to write a sub program with the path to cut the slot and then position the tool to the start point of the pass you want to make and run the sub program. Otherwise I was finding that it's hard to cut from the middle up and down, at the same time, in alternate directions, in the same operation I think you could make a path that's basically a pocketing routine and then trim the excess in tool path edit. You could also do it with a curve/point drive method and draw the lines in there.