Posting to lemmy through mastodon is a thing (hello there!), but yeah, I don’t think you can follow a mastodon account through lemmy unfortunately.
EDIT: I lied! Found the server admin I think at this link: https://lemmy.ml/u/beheerder@social.overheid.nl but I think they need to specifically post to a lemmy server in order to have posts show.
you can get the Delta emulator using the Alt Store on iOS