Yes you can follow channels from other servers without much of a problem. The issue that this on something like Lemmy or Mastodon, that follow would then bring the followed content into the search results of your server. It doesn't seem to work that way on PT.

Example: I have two channels on makertube.net. My husband follows them both, from another PT server (I can't remember which). But just the act of him following doesn't mean my channel shows up in a search from his server.