A turboprop engine is driving a propeller. One can certainly shroud it (see the
Stipa-Caproni as an example). This would reduce tip vortices and might do the efficiency some good, but increase total mass and would not turn it into a turbofan.
A turbofan engine core is driving an enlarged compressor stage consisting of an intake system (reducing the incoming air speed and turning it into pressure), a rotor (adding energy to the airflow), a stator (roughly converting speed to pressure) and a nozzle (converting pressure to speed).