OK, after scouring the internet here's the only choice I see that you have:
Firstly - I don't believe you'd be able to get knobbies through the chainstays and the brake anyway but if you believe that you can:
1) Go onto Ebay and get a set of Mavic Crossmax wheels. (Be sure they aren't disk wheels which have very thin sidewalls since they aren't used as braking surfaces.)
2) Find a set of Hutchinson Cross Comp 25" c 1/3" knobbies.
IF the tires will fit in the brakes then you're in. However I would probably suggest that instead you find something like a small Bridgestone MB (for instance http://cgi.ebay.com/17-BRIDGESTONE-M...QQcmdZViewItem
This will cost you about the same as making a very painful conversion on your wife's good road bike.
Hope it turns out. Let us know what going on.