Spreadsheet has some limited support for modifying an existing Document. This is done by copying verbatim those parts of an Excel-document which Spreadsheet can’t modify (yet), recalculating relevant offsets, and writing the data that can be changed.
Here’s what should work:
* Adding, changing and deleting cells.
* You should be able to fill in Data to be evaluated by predefined Formulas
* Spreadsheet can only write BIFF8 (Excel97 and higher). The results of modifying an earlier version of Excel are undefined.
* Spreadsheet does not modify Formatting at present. That means in particular that if you set the Value of a Cell to a Date, it can only be read as a Date if its Format was set correctly prior to the change.
* Although it is theoretically possible, it is not recommended to write the resulting Document back to the same File/IO that it was read from.