Momo can join the crew. It actually works quite well.
Yes, he wants to be Shogun- that‘s good then, he’s got a dream to accomplish. It would just need to be framed that, through information from the log book or Zunisha, that it is of vital importance for a member of the Kozuki to be on Laugh Tale. That Momo needs to make the temporary sacrifice of leaving his people again in order to fully save them and make sure that Wano is safe. Remember what Hiyori said last chapter- the Kozuki’s repay their promises. If a Kozuki promised Joy Boy to go with them, then that’s what they’ll have to do.
And this would probably also require him to be convinced to do so. Not by Luffy, but more Kine’mon/Hiyori.
(And if Tengu is his grandfather that would be a big bonus).
That would be the argument I’d be going for… if not for Yamato. Yamato wishes to be a Kozuki (Oden, specifically). She’s wanting to really replace her birth family of Kaido with another family of her own choosing- that’s quite a big theme in OP if you haven’t noticed. And, most importantly, she wants to go on and adventure like Oden did while Momo wants to stay in Wano and rule.
So if there’s reason to think a Kozuki needs to be on the crew, she can fulfill that role.
(And if anyone starts wailing about it not counting because she’s not bloodrelated then I cheerfully suggest that you have missed almost everything important in this series.)