Apparently, the Daily Telegraph reports that a 3/5 majority is needed to pass a resolution permitting a referendum. If so, this would probably make it impossible. But although this may be totally clear to a constitutional lawyer, it is not clear to me at all. Article 44(2) of the Greek constitution provides for two sorts of referendums: on "crucial national issues" and on "serious social issues". In the former, government proposals must be passed by an absolute majority in Parliament. In the latter, 2/5 of parliament proposes and a 3/5 majority is needed. But although we could spend a lot of time thinking about the fascinating distinction between "crucial national issues" and "serious social issues", I don't think we need to. The main distinction seems to be between who proposes the referendum. If government, absolute majority. If parliament, qualified majority. Therefore, if we are talking about the former, as I think we are, the referendum is not as impossible as a 3/5 majority would suggest. Unlikely, but not impossible.
It should also be mentioned that when the possibility of such a referendum was first announced - last June, no less - the government also announced that it would introduce changes to several procedural aspects of referendums, which nonetheless must abide by article 44(2) of the Constitution.