To actuality append data to an existing Excel sheet I see this options:
- have the sheet as a data source and the pivot, charts and formats on another sheet. You would have to make sure that there is a new calculation when the Excel file is opened
- use the Continental KNIME nodes to format the data afterwards. I think they are there for this purpose
- export to a CSV or sqlite file and have a database import in Excel but then the file would have external dependencies
If you want just to add a few lines to an existing Excel structure there would be python. But that is not entirely straightforward.
So if you absolutely must provide format the Continental nodes might be your best option.
Date and time formats across different data bases and files is notoriously tricky. You will have to experiment on that. Excel has its own format.
Concerning the database query. I have seen examples in big data environments where it was necessary to always explicitly CAST the columns to the desired format since there were unwanted variations within the large imported files.
Regarding the other question. Yes to read the data from Excel is additional process time question is how long. The other method would be to store the existing data in a table and concatenate that with the new data from the new query.
And a side note: always keep backups of your files ready like in all professional environments since things might go wrong and you would have to revert to a former status. If you have enough space you could work with timestamp extensions to keep older versions.