mate, I commuted on one of these during a very long winter up here. if you're thinking of modifying if for mild off-roading, pick up a cheap XT or something instead. I remember the suspension not being altogether great, even on the road.
I think you'd need to change the mud guards (front at least), which will not be straight-forward, to fit dual purpose tyres, as they're quite tight against the road tyres, no?
sorry that doesn't get you a new set of wheels, but modifying a bike to a purpose it's not meant for gets expensive.
nothing is impossible, but starting from the best possible base is a good idea. I may have got you wrong though, what's yer plans?
btw, if you want recommendations for tyres, post the size of your wheels. many people will not have come across the gt550.