We've been doing that since the dawn of the fediverse. We once federated with Facebook and Google and Twitter for instance and there are still versions of my software which federate with email and to a limited extent RSS feeds. The Nomad protocol (was called Zot) intentionally allows participants from other networks. ActivityPub doesn't. It requires you to have an inbox and a URL for your actor ID and most servers require a public key for fetching things. So there's a higher bar for compliance to be seen as an integrated participant in both networks. This is also why my current work doesn't support Diaspora - it's compatible with Zot but not with ActivityPub.  This is somewhat problematic because you see people "talking to themselves" when they're actually talking to somebody on another network that your software (which I'm assuming is Mastodon) can't see.