custom hotkey

i was wondering if there is a way to have there be more then one hot key for the same fuction?

what i want is to map the shift a to a 4th mouse button, and still have shift a work as well?

all so is there a way to change shift a globally? or do i need to go throught all the modes and set it.
ie in edit mode change shift a to new key, then go to object mode set shit A to ney key… and on.

there are mouse button to key map programs out there but i was starting here.