I'm not at a computer that has construct on it at the moment, but maybe you could do something like this:
If you are using some kind of movement behavior set custom control to YES,
In event sheet:
If keyW is down and keyA is up and keyS is up and keyD is up -> then simulate key_up
If keyS is down and keyW is up and keyA is up and keyD is up -> then simulate key_down
If keyA is down and keyW is up and keyS is up and keyD is up -> then simulate key_left
If keyD is down and keyW is up and keyS is up and keyA is up -> then simulate key_right