..or...you can do this (VOICE DATA MANUAL):

If Strum Mode is turned on, strumming will
be produced when you play multiple keys
simultaneously. This also reproduces the
difference in time at which each string of a
guitar is sounded. The guitar’s up strokes and
down strokes will alternately be produced
when chords are played in succession.
It is effective to play while holding down the
Hold pedal.