You can add a custom function to an expression. If you enter a custom function when you manually create an expression, you must prefix the user-defined function with the namespace the custom function developer provides. When you create an expression with the Expression Editor, custom functions display in the list of all functions and with their function type. Use custom functions as you would any other function.
When you validate the expression, the Designer or Workflow Manager does not validate the custom function. They only validate the expression. The plug-in validates the custom function.