You can actually browse individual profiles from places like Mastodon on Lemmy. They get interpreted as a community where each post is one "toot" from that user.
Though comments and upvotes don't proliferate back to the original platform, they live on your instance.
As far as I can tell, this would subscribe you to a user with the username "fediverse". I've already seen a user and a community both named "cat" on the same instance, so that could be a problem if calckey doesn't differentiate between @fediverse@kbin.social and fediverse@kbin.social.
@Holyginz, I would think you can do like on kbin, since it seems like the standard. Just guessing. Go to the user page on your instance then follow them.