Or, create a new usergroup and then go to ACP -> Group Admin -> Permissions and select your new usergroup and make that usergroup moderator of each forum (select "is moderator" for each forum, like Kevin said). Then, whenever you want to add a global moderator, just add that user to the new usergroup and then they will be moderator of each forum.
That's the thing to do if you are planning on making multiple people global moderators. It saves a lot of work.