Hello and welcome to the fora, Andries.
What are you trying to achieve?
The SPI buffers are implemented in hardware, you cannot change this.
The SPI will always shift out 8 bits at a time with every load of SCID.
Most peripherals that need less than a multiple of 8 bits are aware of this and allow the unused more significant bits to be still sent but all at zero or don't care.
