If a ticket does not have a child ticket already, you can’t assign another existing ticket as a child ticket to that ticket. So one needs to first create a new child ticket under the main ticket to turn it into a parent, then can add another existing ticket to that as child. So, this requires you to create a bogus child ticket, which is not needed.
Linking an existing ticket to another as a child should be possible (This action also can turn the main ticket into parent).