If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
Afaik one problem with Mastodon are the HTTP signatures, because they are using an older version. So theres more than just ActivityPub which might need changes. And just figuring out what needs to be adjusted will take time.