If you have just one SPI slave, maybe you can enable the slave by driving its nCS with a constant.
BUT:
not all SPI slave devices work this way: some chips require to toggle nCS so that the chip can "see" a completed SPI transaction.
Not really...
On a Master you could use any GPIO pin as a CSn, running it in SW mode. But on a Slave, if you want to use CSn (to enable a slave from a master, e.g. selecting 1 of N slaves) - it has to be a dedicated CSn pin for this SPI slave.
You can look...