I don't know of a way to actually change the function of one of the F keys but if all you want to do is provide a short cut to you button use the ampersand (&) just before the letter you want to use as a shortcut key. If you use "Do&Something" as the caption for a command button, then all the user has to do is hit the Alt-S key combination to run the code behind the button.
There's AutoKeys macro which can be used to map custom shortcuts; if you enter "AutoKeys" in Access help, it'll give you a sample of how to map various keys, including function keys to your AutoKeys macros.
