I called the ticket office about this & was told you can add someone as a Friend, but if you don't click the 'allow purchase of tickets' box, it will let you add them.
As the play off final is not a home game, you will apparently still be able to buy a ticket using their reference when you get to the basket. If it won't let you do it with the drop down menu of your friends/family, I was told you can just search using their client reference in the basket.
The ticket office staff were able to tell me all this straight away & with confidence, although I must admit I'm nervous if it'll work!