I think a good starting point would be advertising Lemmy where possible on your subreddit, such as pinning an equivalent futurology community from Lemmy linked to a reddit post and pinning that, also mention your lemmy community on other places like mentioning Lemmy and asking users to consider Lemmy instead on the welcome message when new users join/subscribe to r/futurology.

Kbin has this figured out, they now have an in-built community request system.