Modifying the Excel rendering is not supported through the viewer's API. Events can only point you about the current operation in the viewer and its state. In next releases we plan to extend the arguments with details about the document state, where it is saved and others.
Still working with the already produced documents is not supported by the reporting engine and you will need third party libraries to modify the files. They will be rendered by the selected rendering extension based on the listed design considerations - Design Considerations for Excel Rendering
Let us know if we can be of any further help for you.
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