Conversion SSI to SPI
You would need a converter board, as the SSI isn't just hardware layer protocol, it has also a protocol layer. If you are doing serious things, then you might be interested in SoC or MCU with these protocols already implemented, like Renesas Cortex R4, TI Sitara,...
EDIT:
Note on SSI and MCU
As from note, the SSI can be implemented with SPI pretty straightforward. Perhaps you would need an isolated RS422 transceiver rather than using MAX IC. The app. note describes hot to use DATA galvanically opto isolated and CLK non-isolated. Nowadays you have isolators and PSU all in one IC, see MAX14858 or ADM2582E for example.