Absolutely do it! I added on as AU on DH's almost 19-year-old Discover, and even without the credit limit, the history was great for my EQ and TU scores. (I have one older account on EX alone.)
There's no telling when FICO 08 will finally stagger into action --maybe next week, maybe next year, maybe never. And even when it does, each individual lender will have to decide to make the switch, so that AU account should help for an indefinite period of time. In the meantime, keep working on your "real" credit picture, and use the increased score to improve what you've got.
I've decided not to switch over to joint until my back is up against the wall. What I have is working fine for now.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007