Leagues have increased their sizes before in the offseason. It may require hitting me up at help@ottoneu.com, but my recommendation would be to really aim for starting with the number of teams you want in the league rather than adding more later.
A few people have tried doing expansion drafts:
Ultimately expansions are really rare (for a number of good reasons) so I’d try my best to avoid planning to have an expansion draft in year 2 before even starting.