If you think you had to be an existing SkyDrive user to take advantage of the offer for additional space, that doesn't seem to be the case. I know I never signed up for anything but a Hotmail account (which, actually, I never use).
When I signed in just now, I noticed a SkyDrive link at the top of the page. Clicking on it and selecting "Storage Management" allowed me to upgrade to 25GB, even though I had never used the service (or asked for/enabled/subscribed to it).