There is a PWM available in KiCad/ngspice as a code model. It has been used in several of my examples at More simulation examples for KiCad/Eeschema/ngspice, for example in the Regulated boost converter or the Digital PWM amplifier.
ngspice supports direct simulation of digital Verilog blocks, which have been compiled with Verilator (see Ngspice, the open source Spice circuit simulator - Special features). This feature has not yet been used in conjunction with KiCad, but I don’t see any roadblocks.