Are you wanting to connect the keyboard to the amp wirelessly? The text of your post doesn't say that, but the subject line does.
The PSR-S900 can use a wireless connection, but that's for internet access over a wireless LAN. As far as I know, the keyboard cannot transmit its audio output over a wireless LAN connection.
On the other hand, I suppose you could buy a wireless audio transmitter and wireless audio receiver. You'd need to connect the keyboard to the wireless audio transmitter via audio cables, then connect the wireless audio receiver to the amp via audio cables.