I never shrink my batting before quilting, because I love crinkly quilts.
But I have used my washer to soak and spin without agitating, here's how I did it:
Filled washer to top, then unplugged it. Let it sit overnight. Next morning I plugged it back in, then hit the cancel button, then the drain & spin button.
I recommend you experiment with towels or something first because you do NOT want to agitate the batting. You'll get an unusable mess.