Contiki: Serial Line IP via SPI
SLIP is a protocol for conveying IP traffic over a serial line; it sits one layer above UART, SPI, etc, in the OSI network stack.

SLIP over SPI should be largely the same as SLIP over UART, except for the need to specify who is sending and who is receiving, and potentially a way for the slave to signal when data is available. Your part's datasheet should specify how to do this.

Edit: However, it looks like you might be talking about something called 'slip-radio', in which case you're not sending SLIP over SPI: slip-radio talks the cc2530's native protocol over SPI and converts it to SLIP to communicate with your host PC.