Some of the machines in our local gym have removable plates that fit over the center pin and stack into nubs on the top plate. I have borrowed these from other machines before to get more on the stack. If you do borrow be nice and put em back.
Also be careful to not overload. The designer of that machine might not have put a thick enough cable on it for a big increase. My rigs cables are rated for 600# but my partners rig can only do 400#. Wish I was doing enough to snap either of em. Heh.