pwm after constant current switching driver
The XL Semi XL4015 5A 180KHz 36V Buck DC to DC Converter does not have a PWM (or Enable) control input. 



It is very common to add an external MOSFET to a converter chip with an internal MOSFET.  You should be fine.

You would add your MOSFET to the output of the SW pin.





I think it would be easier to build this Diode Inc AL8862 circuit than to retrofit the above circuit. The CTRL pin is your PWM input.