How to add iPhone to my product list if it's already registered to another apple ID?
As far as I know, an iPhone is linked to an Apple ID until it is released by the owner or by Apple techs (like when your phone is traded for a new one, rather than being repaired and returned to you). 

This is intended to prevent theft; you can't just take someone's phone because they have to release it from their Apple ID first. If they don't, the phone will be very trackable by the owner of that Apple ID, the phone can't be reset without the owner's Apple ID &amp; Password, and you can't get any new apps or anything because you won't have the Apple ID &amp; password to enter for a purchase or to enter in order to remove the Apple ID from the phone.

If you got this phone legitimately, you'll have to contact the "tech. service" you used and ask them what they'll do to fix the issue. They may be able to take it back, and, as an authorized Apple repair center, send it to Apple to have the Apple ID cleared. I'm not sure they would or could do that.

There's nothing a user can do to release a phone without the original Apple ID &amp; password, and this is intentional. If Apple set up 'online support' and allowed any user to release any phone from the Apple ID associated with it, then it would be much easier to steal and sell iPhones.

...again, as far as I know... ;)