Hi David,
As Demian mention, the only other way (and recommended way) to add your custom Java code is via a WorkItemHandler implementation. For that you will need a Java class. Once you have your java class you will need to create a Jar file (container of compiled java classes) and add that jar file to the jbpm console class path in order to use it. This means that the jbpm console needs to have access to your compiled class, in the same ways as any other application.
This is a really common way of accepting extensions for any other java application, so it should be simple for you to create that.