I'm a little unclear about the process for moving an outline previously
started into a more recent outline.

I started a contact list and later decided I wanted those numbers in
a page with friends and family.

Can I "import" one opml file into another?

I've heard the word inclusion but I'm a little shaky on whether that's
actually what I need to do.

PS. I know I can save an opml file to my desktop and can also import it from there, but the
question is how to get it actually "inside" the first outline?