RadExpressionEditor supports only its own custom expression language grammar that is somewhat similar to Excel's formulas. It is not possible to plug-in other grammars, because the purpose of the control is to allow manipulation of expressions in their free-text form and provide a LINQ expression translation as a structured data on top of the given expression.
I believe that what you are looking for is some kind of domain specific code/text editor that will allow you to edit your custom workflow definitions. Currently we do not have such a component in our portfolio, but if the need for this kind of control grows we can think about providing a solution in the future.
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