Making an adapter as you speak of probably wont work.
On the other hand, your Deck already has the ability to be converted from PS2 to USB and vice versa. Basically the cable that is used on a Deck Legend is a PS2 plug on one end and a USB plug on the other. If you are using PS2 now and want to switch to USB later all you need to do is flip the cable and clip out one small resistor (alternatively if you go from USB to PS2 you need to solder that resistor back in). For more details, please see our user manual on this page:http://www.deckkeyboards.com/customersupport.php