You can't on Lemmy because content outside of communities doesn't federate. But outside of that, most Fediverse platforms let you follow Lemmy communities and users, alongside everything else on the Fediverse. I can only recommend doing that on kbin and Friendica (which I'm on), though, since most other platforms don't federate Lemmy's post content.

But I still regularly catch a Mastodonian in the Lemmy comment section and occasionally even a post from one (They post here by tagging the community, by the way). @aumalatj here below you is a Mastodonian, for instance.