Using the Pico as a SPI _SLAVE_

[rjnbarrass]

Hello
Due to wanting to use a PC as an emulator of a production device, we want to be able to connect to the same connection as our host device (which is a SPI Master)...

we would like therefore be able to connect the Pico acting as a SPI slave, and the (UI based on C#) communicate via the PICO as that slave.
(we also use a GPIO as a flag to the host so the GPIO support will be used for that)

Is this possible - i think currently, the interfaces supported on the Pico is a Master implementation of the SPI only - but wonder if we can extend and create a SPI-slave.cpp implementation?
regards