One of the limitations of the Tyros instruments is adding new sounds, (Unlike others Yamaha doesn’t allow Akai samples etc to be loaded) however here is a program (It may have been posted before) that allows Yamaha owners to convert other samples into its own format, (As with all converters some work will need to be done by the user after the conversion has taken place) thus allowing uses access to the vast number of sound samples on the market.
http://www.extranslator.com/formats.html

Regards

Bill