Without understanding what software program we are talking about this is a bit hard to answer. Most software programs use F-keys, entering menu names on command lines, etc. to control navigation through your application. Just creating a hot-key on your keyboard will not change the way the program works. If you have to press F12 to go to the previous screen it seems useless to program a Ctrl-B on the keyboard to do the same when pressing F12 will suffice.
Some applications might use a command or instruction string to go to a previous screen. If you are using Client Access to get into your iSeries, you could create a macro with the command string to execute and assign the macro to a Ctrl key. When the user presses this key, it will type the command onto the command line and execute it.