Yes, you can purchase a phone at any time without adding another line. There have never been any requirements to make a phone purchase.
If you are asking if you can get a buy one phone, get one free then the requirements for that promotion are that one of the devices has to be on a new line of service.
Additionally, you should never post your email address on this public forum for all to see. It is against the terms of service and may increase the amount of junk email you receive.