I don't believe that Yamato will join simply because she probably knows what One Piece is and all the other stuff about Laugh Tale
Yo but imagine this though
Yamato offers to tell the crew what she knows, and they decline because they all want to find out for themselves, so she stays quiet about it
Then when they reach Laugh Tale, whenever we get a big revelation about something Yamato is just like "yep, knew that already". Kinda like this moment from ch. 603
!