I inquired a few years ago (1992) (via snail mail) and got a letter back from:
Republique Hellenique
Ministere des Transport et des Communications
Administration des Postes det des Telecommunications
49 Avenue Syngrou
GR - 11780 Athens
Greece
saying, paraphrased, according to Greece Legislation and the agreement of reciprocity between the USA and Greece, our administration may grant you the following:
A. A provisional license for a year. Your call sign during your stay in Greece will be SV/ your international call sign /P or /M.
B. A radio amateur's licence for three years with new call sign SV0.
An application needs your name/address, type of equipment, time and place you are going to operate and a copy of your American license.
I was assigned SV0JG when I was on Crete with a Greek form with handwritten information and some stamps on it.
This was a few years ago, so I'd first start by writing a letter of inquiry with the above information to the above address and a copy of your license and see what comes back.
You may want to contact someone in the Radio Amateur Association of Greece first and ask questions. Email today makes it a bit faster, if they respond...
http://www.raag.org/homepage.asp?ITMID=2&LANG=ENIn the Licensing tab of the above web page, it says:
For Applications and Information Contact:
Mr G.Velonis (g.velonis@mindigital.gr)
Ministry of Digital Policy,
Dpt. of Communication Control
FRAGKOYDI 11 & ALEKSANDROY PANTOY
P.O. 10163
KALITHEA
GREECE
Telephone: 0030- 2109098879