1. Open the SalamanderGrandPianoV3.sfz file in Polyphone.
2. There should be 12 instruments. If so, open 'Presets' and open '000:000 SalamanderGrandPiano'.
3. Remove 'SalamanderGrandPi-11' and 'SalamanderGrandPi-12'. Don't forget to remove unused elements!
4. Open 'samples', and select the samples from 'rel1L' to 'rel88L'.
5. Click the button next to the 'loop' function. The 2 numbers should stay '0' and '0'.
6. Now, change the second 'loop' number to 1.
7. Open 'instruments'. Once you have done so, click 'SalamanderGrandPi-10'.
8. Set the loop playback on the global section to the third option.
9. Repeat steps 4-6 for the samples that begin with 'harm'.
10. Repeat step 7 with the instruments SalamanderGrandPi-3,4,5,6,7,8 and 9.
11. (Optional) Go back to 'presets' and open '000:000 SalamanderGrandPiano'.
12. (Optional, continuing from step 11) Change any key range that has 20 or 21 as its low number to 0 to the same high number. (If this sounds confusing, here is an example. Change 20-43 to 0-43, and 20-88 to 0-88. Keep on going.) After you have done this, change any key range that has 108 as its high number to the same low number to 127. (Example: Change 89-108 to 89-127.) This will make the soundfont play all 128 keys.
13. (Optional) Normalise all samples to 95-98%.
14. Save the resulting SoundFont as anything you like.
I originally wrote this up to help someone on the Polyphone website forums.
NOTE: If an sfz file is saved, click 'Export soundfonts' instead of 'Save' or 'Save as...'. On the 'Export...' menu, set the format to sf2 then click Export.