I just want to add that if your packs are in PPF format, you can also make a new PPI file for your SX900 in the YEM tool (after you have imported the n27 file of your SX900 into YEM).
Other formats (CPF/CPI/PPI) created specifically for the S770 cannot be used on the SX900: you will have to ask the supplier to deliver you a new file or as explained by Fikke: in case of packs created by Yamaha, you can download the SX900 version for free from the Yamaha store.