Based on your question about PDI, the closest entry point would be an "User Defined Java class" step.
References on: User Defined Java Class - Pentaho Documentation
You can develop somethigs similar to a POJO but it will be automatic modified with pentaho stuffs and compiled with janino.
Or you are talking about messing with the PDI source, in that case, references: GitHub - pentaho/pentaho-kettle: Pentaho Data Integration ( ETL ) a.k.a Kettle