You do know that a "Spreadsheet file" is just a text file, don't you? So you could simply read it as a Text file (say, line-by-line) and write it out with another name (at least while testing, so you don't "destroy your data" while your testing your ideas). If you do this, can you figure out how to add a single value (a) at the beginning of the file, or (b) at the end of the file? [This is really a very simple question -- don't "overthink" it].
Bob Schor
P.S. -- a Text file means that the data in the file consist of strings, usually separated by "line-terminator characters", most typically the character pair <CR><LF> (Carriage Return, hex D, and Line Feed, hex A). Check out how Read from Text File works. I'll note that the Help documentation is unclear about how to turn on and off "Read Lines" -- right-click the function on the Block Diagram and click "Read Lines" from the Drop Down to change the function.
