Hmm, only thing you can try is to add this line in build.prop (/system/build.prop):
Code:
qemu.hw.mainkeys=0
But I have read it has no effect, too. No Idea, i think it is impossible to add on screen buttons (except mod, but don't know one). But why you want it
EDIT:
Have to wait for Sense 6.0 Port (if possible)