Make a 4 wheel drive a dually?

I have a 1989 ford f-150 four wheel drive and was wondering if I could turn it into a dually

You could. At one time there were places that sold kits. Not sure if they still do or not. If you really like the dually look why not just find a 4x4 dually, they are not rare unless you are down south.

Replace the rear axle and add some fender flares

You'd need the axle from an F-350 and probably the bed from one as well. Ford never made a dually F-150 as the other components of the F-150 like the brakes, suspension and transmission weren't up to the task of handling the kind of weight that an F-350 dually was made to handle. From a functionality standpoint it would be foolish make a dually out of an F-150. It just screams "poseur" to me.