Hi furry 1281,
You can use soundfonts (.sf2 format) in connection with the Yamaha Expansion Manager (YEM):
Either you import a .sf2 file with "My Packs > Import Pack" (this creates a new Pack with the voices of the .sf2 file), or you first create a new, empty Pack ("My Packs > Create Pack") or use an existing (not protected) Pack and import the .sf2 file with "Import Content".
Note: There is not 100% compatibility for all .sf2 files. You may have to rework the Voices in question in the YEM. (Double-click on a Voice in YEM opens the Voice Editor.)
Of course, the new pack in question must finally be installed on the instrument (in your case on the Tyros5) as usual. It should be noted that when using the "USB stick method" with the Tyros5, the expansion memory is always reformatted, i.e. all previous expansion contents are DELETED. You can only avoid this if you use the "WiFi method" for the Pack Installation (in connection with the USB WiFi adapter "UD-WL01"). However, it can happen from time to time that the Expansion memory is formatted anyway...
Best regards,
Chris