If yer talking hardware addons you could always pick up a Leslie Simulator Pedal like the Neo Ventilator II to add some crunch to the organs. Personally I'm using a Muza FD800 effects pedal and a Behringer RM600 Rotary pedal with mine.
If yer talking actual hardware rack modules the sky's the limit depending on your budget.
You could pick up a used Triton Rack or Moitif Rack for about $500 on Ebay or buy a brand new Roland Integra Rack or Muse Receptor starting about $2K