At one time I was told of a 3 or 4 letter acronym (not ActiveX or anything like that) for a technology which allowed writing to an open Excel document. I searched for that acronym on Discussion Forums and found an example which perfectly addressed the problem I was having.
Unfortunately I can't find that example anymore and I don't remember what that acronym was! Any ideas?
As long as the file is not locked you can write to it using either the report generation toolkit for ms office (labview add-on) or using the ADO interface (sql) using the database toolkit or other ado db tools written that are available for labview. Search the forums.