One way to do it is to follow lbattraw’s suggestion and use a bitbanged spi bus. There is an easy and hassle-free way provided by
https://dev.openwrt.org/browser/trunk/package/kernel/spi-gpio-custom/src/spi-gpio-custom.c
Quick and easy.
You could also multiplex other devices on the hardware SPI bus, but that requires a kernel driver and custom machine setup code so that the SPI bus is shared properly.