iPhone Upgrade Program without adding line or upgrading? Is it possible to do the iPhone upgrade program without having to either upgrade an existing line or adding a new one?  Just doing it for monthly payments?
No, it's not possible.  You're required to either upgrade an existing line or add a new line through the IUP.  

You don't have to use your existing cellular carrier.  If you have, for example, AT&amp;T and want to use T-Mobile for a specific iPhone purchase, you can do that.  However, you need to select a carrier for the iPhone (even though they're sold unlocked).