Unless the controller is also the met observer, he/she should not have to make ANY corrections; the met observer should do this prior to displaying the QNH on the controller's display's.
Where there is no qualified met observer, any QNH passed by the ATSU (whether ATC, FISO or A/G) will not have any correction added, but in this case the airfield will not have a published instrument approach procedure; if a pilot chooses to design his own private procedure, then it's up to him/her to work out any correction necessary.