Expressions are java classes. You can develop your own expressions if you want to enhance OpenWFE's process definition language. You have to extend at least the FlowExpression class. Then you have to add the expression to the expression map (etc/engine/expression-map.xml) with its short name.

Of course, the next step would be to enhance Droflo to enable people in your organization to use your expression when designing processes with Droflo.

For more information, see Chapter 12, Extending OpenWFE's process definition language.