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.
Something which works similarly to Lemmy or Reddit is the obvious answer, such as littr or lotide. But afaik neither of them has federation anywhere close to ready.
But in fact there are so many things that need work on Lemmy itself, that we dont have much time to work on federation with other software (unless devs from the other side are also willing to get that working).