No. The biggest reason is that the 732 sits much lower to the ground than the 733-4-5. Because of this, the engine nacelle on the 733/4/5 is flat at the bottom (you've noticed). They had to extend the landing gear to make the plane "taller" even with the flat bottomed engine.
The underbelly/wheel/wingbox area would have to be replaced.
When you get down to it, there isn't much point. Since 737s are still manufactured, just buy a new one. With 727s, it becomes a sensible thing to do.