Use a pitch shifter(s). In a different multifx modelling device I add a -12 and a +12 to get a basic organ sound. You can add +5 -7 to fatten it. Some phaser or swirling speaker can help give it some movement while reverb helps to smear out the sound. Using a Boss Slow Gear style fade in effect to remove the pick attack sound from the pitch shifted sounds helps alot.
The EHX Hog2 is very good at making organ sounds.