Thank you for the answer.
Example: (app.Layer is a FeatureLayer)
//adding a new Field
for (feature in app.Layer.graphics) {
var attributes = app.Layer.graphics[feature].attributes;
attributes["DIFF"] = <someValue>;
}
Debugging app.Layer.graphics.attributes shows "DIFF" with the contents <someValue>
//here creating the ClassBreaksDefinition, GenerateRendererTask
//here Comes the error: unable to find classificationField
generateRenderer.execute(params, applyRenderer, errorHandler);