At the moment Divshot supports Compass but not Susy. You can include Compass in your custom stylesheet like you normally would:
@import "compass"
Your CSS is sent over to Alloy, our web API for CSS preprocessors. You can take a look at the gems we're using here: https://github.com/divshot/alloy/blob/master/Gemfile. We'll consider adding Susy in the near future!