Can a Panel be converted to a Mini-Panel?
Yes and no.  Panels is a module with a whack of facets.  Based upon the context of your question it sounds like you created a page via the panels interface.  So.  That's a page and that's why you can only really link to it ( or override another drupal generated page.  

Mini panels are exposed as blocks and also as elements usable in other panel facets (like pages or other mini blocks).  

You'll have to create a new mini panel from scratch but all the elements you used to create that custom page are available to you.  If you made custom elements just check the make reusable and then they'll be available to your mini panel.