"Crossing over" is only achievable by creating another account; there's a way to link them on your profile. I don't see why you'd want to, though, other than to view the Local Timeline.

However, you can interact with people and posts all across the Fediverse simply by looking at their posts and trying to interact with them. Don't click log in; you're not trying to log in! The software prompts you to log in, and you can then log into the place where you've got an account. Example: Look at this post, and try to interact with it; it'll prompt you to log in, and you can log in with fosstodon.org.

The update you were talking about made this a little easier.