(Edit) The only way to set G54 with the current machine coordinates is to use the AutoLoad feature in F4 screen. I do this after every work offset probing sequence.
Would have been ideal if we could use numbered parameters to set the G54...G59 offsets (eg. #5400...#5900) and other user set values in memory (eg. #1001...#1101). That way we could program the G54 offset on the fly after probing and create a generic G-Code for a standard design then simply adjust the parameters for a given part or pattern. I believe thats how its done for most programs that have conversational programing. You enter the value in a window on the screen and the G-Code program calculates the move based on the values you enter.