If you take a look at the official documentation you will find a section that is called Domain Specific Activities or something similar. That section explains how to create your own work item handlers. Because workItemHandlers are just classes that implements an interface you can extend them if you need that.
Once you get your handler implementation you can register it to the session using ksession.getWorkItemManager().registerWorkItemHandler("<id>", instance);
In your processes you can use the Task node to hook up the workitemHandlers that will be used in runtime:
<task id="_1" tns:name=[id]/>
Notice that the [id] is the same as you register in the workItemManager.
Cheers