Can't you create a new wall type and then use that when adding the wall?
I wanted to create balcony floors outside of recessed upper floors---so to create a floor I made a balcony wall type with a .25m height...and then created the outer
wall of the balcony with that wall type---it had height of .25m, while the remaining walls on that floor used the 2.438 height.
So--I THINK you could create a new wall type on a story, then select that when you create your wall....
So most of your walls use the DEFAULT wall---but when you draw this particular separation wall you'd use your newly defined wall type....seemed to work for my need.