Hello @barp welcome to the KNIME form. You could check out these discussions. There is no genuine way to just export the data from KNIME to excel and keep the formats but you have some options:
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 …
Well yes that is possible. You could use the mighty Python library OpenPyXL . Yes it is some work but it is doable (you have a few links to examples attached).
Then there are the Continental Nodes for KNIME that deal mainly with formatting. They might be useful and possibly easier to handle than Python.
Python Excel Drop Down
Python Excel manipulation
Python Excel table object