SPI Slave on Arduino Due

Hello,
I am actually working on a quite complex setup with an Arduino Due as centerpiece. The SPI bus is used with the Due as master to talk to some external ADCs and DACs.
Some outputs are used to shift out serial data through shift registers and now I need to find a second SPI bus where the Due is slave, so gets the SPI clock from somebody else.
I have seen that in theory the USART modules of the SAM3X8E can be used as SPI bus, but in the Arduino environment I am not feeling very comfortable with configuring the PIO.
Do you have an example code where you basically set up a second SPI in slave mode please?
What reception model do you choose (polling, DMA or interrupt on receive)? I guess that the SPI can only receive byte by byte so I need to get the data fast enough to not loose anything.
Thanks a lot.
Cheers
Felix