Searching for it is a valid option, it's basically what the boost/federate not does if I'm not mistaken.
Searching for it adds the community, but not the content.
If someone subscribe, we'll receive new posts, but not the existing one.
Syncing existing content can kinda be done by searching for each original post's permalink.
I've done a few posts manually, so they'll show for us on SJW.
Syncing existing comments would probably require having some sort of link for each comment which I'm not sure where to find on wordpress...