Hi AJ,
please read the section "Applying Voice Effects > Applying Harmony/Echo to Your Right-hand Melody" in the S670 Owner's Manual, pages 40/41. The needed function in this case is "ECHO Category > Tremolo".
Make sure to also refer to the section "Making Detailed Settings for Harmony/Arpeggio" in the S670 Reference Manual, pages 5/6.
P.S.
Some Voices are already sampled with the "Tremolo" playing style. If such a Voice is programmed with a Velocity Switch (the voice plays either normally or in a tremolo style, depending on the velocity), you can go to the VOICE SET and use the parameters "TOUCH SENSE DEPTH" and "TOUCH SENSE OFFSET" to adjust the "switching point" at which the voice changes to play the tremolo (see pages 12/13 in the S670 Reference Manual).
Hope this helps!
Best regards,
Chris