Why would you want to do that ? The best way to copy confusing HEX code is to highlight the characters using the mouse, and right click and select Copy. Then paste the result into the (presumably wireless adapter's) user interface.
I guess you're using WEP (and 128 has ridiculously long passphrases) -- if possible I would switch to WPA which is simpler and more secure.
Well, mr. smartypants, one VALID reason is because some machines (EeePC for one) get confused and think that some ASCII keys are HEX, when they aren't! So you HAVE to convert the ASCII key or just not get connected. I'm sure there's a converter somewhere, but it might just be easier to do it by hand. :-P