Opening layers directly from XLS spreadsheets is a very useful (if not well documented) feature of QGIS, but I can't seem to be able to properly import cells where the contents of the cell is the result of a formula. The events are:

1. Set up an XLS spreadsheet where one column is the calculated result of others (e.g the sum of the other columns), the 'sum' column
2. Import spreadsheet as a table into Qgis by using 'add layer' and selecting the XLS file and clicking open.
3. The table loads up with all the correct values, including the calculated values in the 'sum' column
4. Save the table
5. Return to the XLS spreadsheet and your 'sum' column no longer has any formulas, it's just the integer values.

Is there anything I can do in the importing or saving of edits that will preserve the column as a formula, not just the results?