So you mean like subscribing to the entire instance of lemmy.ml and only having lemmy.ml in your subscription list and not the induvidual communities? If so, no, you can't do that. But you can browse using the all button at the top of the page next to local to view all posts that is from federated instances :)
I was thinking it's more like subscribing to all communities of an instance, because the instance I'm interested in is kinda small and not a general one. Would love to have an option to view all posts from a certain instance tho