The answer is more complicated than you think. If your FAA license is unrestricted, as in NOT based upon a foreign license, then you can NOT add to it unless you do the flight test. You would be issued a separate license based upon your foreign, thereby giving you two licenses.
If however the license you hold is based upon a foreign license then you can add to it.
See FAR 61.75
http://ecfr.gpoaccess.gov/cgi/t/text...0.1.1.2.2.1.10