Tye, as Dave correctly stated, there's no way (on the Tyros & I believe the PSR3000/2000 as well) to assign 'pitch bend' to keyboard aftertouch. The only way to produce (while playing live) a pitch bend, is from the pitch wheel itself, or assigning pitch bend to one of the Tyros' three assignable footpedals.

Function: Controller > Foot Pedal Tab

Foot Pedal 1, 2, or 3:
Function > select 'Pitch Bend'

Scott