Not only is what you say perfectly possible - you can also have people from Mastodon talk to you!
This is my Mastodon account. I've followed a couple Lemmy communities and I'm seeing their comments in my feeds, just like they were "real" Mastodon posts.
It doesn't work the other way around (so you can't follow me in Lemmy, for example), but it's pretty neat that it works as well as it does!