I stand corrected - being too hasty. The custom field can only hold a single line. So my advice was unfounded. Of course you can delete the current text and put your location there instead, but not both concurrently.
However, do NOT despair. Andrea ('annedi') will turn her prying eyes on the scrips used at SMF. There is already a location field when people register, so the challenge now is to get it displayed in the forum messages.