The mechanism you want is the AttachTo<> function built into the WatiN Framework. Since WatiN only supports a single-threaded apartment state, you have to manually manage your session(s).
The link above has a code example for using 'AttachTo<>' but, for the sake of being thorough, I have provided one below as well.
var IE = IE.AttachTo<IE>(Find.ByUrl(someURLHere));