Software SPI is now supported on AVR, Due, and Teensy 3.x boards.

The most general approach is to set USE_MULTIPLE_SPI_TYPES nonzero
in SdFatConfig.h.
//------------------------------------------------------------------------------
/**
 * Set USE_MULTIPLE_SPI_TYPES nonzero to enable the SdFatSoftSpi and
 * SdFatLibSpi classes. SdFatSoftSpi uses software SPI and SdFatLibSpi
 * uses the standard Arduino SPI library.
 */
#define USE_MULTIPLE_SPI_TYPES 0


You can also edit other configuration options in SdFatConfig.h to
change the type of SPI used by the SdFat class.