Hi Pietro:
Yes, you can port MFS (MQX file system) to SPI flash. But we don't suggest our customers do this. MFS is not intended to be used on internal flash memory storage, due to wear leveling reasons. MFS updates the FAT tables very often, and the FAT table is stored always at the same location, thus, the flash memory wars out quickly. Eevery update to the FAT table would require to read the flash sector, modify the data in RAM, program the data from RAM to the internal flash sector. The maximum number of Program/Erase cycles of an internal flash sector is limited.
Regards
Daniel