Allowing for extensions to the current ActivityPub/ActivityStreams vocabulary is one of the tenants of the specification. Nothing says everyone must implement everything.
I don't see how this could be done in a backwards compatible way.
How would incompatible servers know to trust a server foo.com to publish posts for an account bar.com? How would they know where to look for bar.com's posts when their users search for it?
How would incompatible servers know to trust a server foo.com to publish posts for an account bar.com? How would they know where to look for bar.com's posts when their users search for it?