... Big pic: http://i.imgur.com/EFOSgAZ.pngHow about dragging the desired parameter from the main GUI and dropping it into the parameter list? As in this mock-up:
... Big pic: http://i.imgur.com/YQY6N0g.pngOr even dragging the desired parameter from the main GUI and dropping it onto the Multiparameter button (then opening it to Edit the details). As in this mock-up:
... Big pic: http://i.imgur.com/cFyo4SC.png