Hello
You will have to create a Workflow class instance. The instance should be unique (you can select the primary key of custom tables entries). You can create a Class with static methods (refer CL_SWF_FORMABSENC) and call them as step in the workflow . You can include the custom code in these methods
You can refer to following document for an overview on linking the custom tables with Workflow using change documents.
Triggering a Workflow for a custom Change Document Object linked to a custom Business Object
Regards
Sandy