If the built-in subsetters and processors in SOD don't meet your particular needs, external processors may be the answer. They allow you to write Java code that hooks into a SOD recipe at any step with the same information and ability of the built-in code. Writing an external processor requires some additional skills beyond those needed by a regular SOD user. Your life will be much easier if you've written and compiled Java code before.
The externals tutorial covers creating a simple external origin subsetter from soup to nuts. You should start there to get an idea of how a processor is constructed. After going through the tutorial, check the various types of externals to see which one fits the bill for your needs.
If you run into some snags or need a starting direction for your own external processor please contact the SOD Mailing List for help.