Not sure if this is what you are looking for, but I went to Mastodon, grabbed my full profile @name@server and searched for it from the search bar on the top, then clicked Follow.

It isn't reposting or crossposting, but I see my Mastodon posts in my Circles / Feed here.

Short answer and unsatisfying answer, pay for it. It's not really a functionality that exists within the site, to my knowledge.

fedi.tips/crossposting-between…

However, you can do it manually on here by searching for your mastodon handle with an @ in front of it, following it, and then resharing anything that the account posts. Not automatic, but free.