No hack needed.
This option is already available. Go to your control panel -> Members -> View/Edit Member Record. Then type in the member's login name. Click on the name; another window will pop up. From there you can grant access to private forums for this member.
Oh, I forgot... This is only for Admin., not mods.
I see what you're asking for. However, this should be a simple hack.
[ 11-02-2001: Message edited by: freeTV ]