if you have the Office 2003 with the greek proofing tools you can install it for the Office 2007 as you can see here:Some Office 2003 proofing tools may not behave as expected in 2007 Office programs

and here is a guide:
Mike's Blog : Install another Office 2007 Proofing Tools (e.g. Dutch)

it is not an official guide, but a user there reports that it works with the greek proofing tools.
The official greek language pack for Office 2007 may take long so if the Office 2003 proofing tools don't work for you, you may have to ask support from microsoft.
Good luck!