Are you sure they're only local videos? If I visit some instance like peertube.wtf it's mostly external videos both on the home page and the discover page. It's just that Peertube doesn't add the domain name to the usernames or descriptions in the feed, so they might look pretty much the same whether they're from remote instances or local videos.

I've been on Peertube for some time, so I have a good amount of subscriptions from all over the place. And I believe you should be able to subscribe to any channel, whether they're on a federated instance or not. As long as they're not blocked.