You should be able to search them in the same format as you'd search for federated Lemmy communities. For instance, [!askkbin@kbin.social](/c/askkbin@kbin.social)
should let you access Kbin's version of this community (assuming I know how to format the text correctly).
From there, you can subscribe to it from within your instance on lemmy.ml, and it'll show up in your feed as the rest of your content.