A year late to the party, but here is almost the answer you wanted to hear: Yes, you can do USB-PD on processors that do not have a UCPD Module, though you can NOT do it on your existing board.
I prototyped USB-PD communication on a STM32L chip, before I was able to find stock for STM32G0B1. It is hardware and software hacking at its finest and not for the faint of heart, but it can be done. You have to write all your own code and need a bunch of external hardware. You will likely not be able to meet all the specs spelled out in USB-PD 2.0 which means you most probably cannot get it certified, but you can get it to work.
To transmit: Compute and append the CRC. Convert binary to 4b5b while adding the preamble and EOP. That gives you BMC bitstream that can be sent out using the SPI port.
To receive: Have an external fast comparator with adequate hysteresis convert the 1.2V signal into digital stream that is fed into a timer input on the processor. Use input capture with DMA to collect the transition times for all the edges. This is your BMC data (lots and lots of RAM). Software can decode that to verify/strip the header and EOP and come up with the 4b5b data. Convert the 4b5 to binary. Check CRC.
Not something any sane person would do if you could buy a processor with UCPD module. But these are not sane times and the unavailability of chips is driving many of us insane :)
Regards,
Hamid
