Yes, from a technical point of view it should be possible but most roaming partners make their own agreements when it comes to incoming and outgoing calls.
Also, the info on their page could be incorrect but I doubt it. For example, for their partners in Germany they state that you can receive calls/SMS and have GPRS/MMS on all four German networks but you can make outgoing calls and 3G/videocalls only on O2 and T-Mobile.
Their above info is correct for all four German networks. When I initiate my Wind prepaid card in Germany, I receive automatically two SMSes, the first states the prices for calls/SMS/data when roaming and the 2nd says something like
"Additionally you can make outgoing calls from the O2 and T-Mobile networks".
edit: I checked also their non-flash roaming pages for prepaid customers, and they have the same info like the flash one. For Canada you can receive calls/texts and send SMS on all three networks but strangely you cannot make outgoing calls.
Roaming partners for incoming calls/SMS and outgoing SMS:
http://www.wind.com.gr/Listing.fds?t...=1334&langid=2
Roaming partners for outgoing calls:
http://www.wind.com.gr/Listing.fds?t...=1344&langid=2