start with a new wheelset... same number of gears, knobby tires on your second wheelset...
that way you can swap out when you want to go cross and swp back when you want road...

If its just to change your bike to ride on trails and road... just buy new tires...knobbies that arent too big, probably a 700x30, but check with your Local Bike Store to see what your tire clearance is. Also you would probably need a 1 1/8" threadless cross fork with cantilever brakes on it... not tough to mount But that all depends on how safe you feel off roading with your calipers...


Maybe down the line you can consider investing the money in upgrading the parts you need... but at that poin I would just suggesting o buy a cross bike instead of trying to convert a road bike. Might be cheaper.

cheers