You might be able to leverage the XLS odbc functionality and set your FM DB a data source.
Get ready to master VBA / Macros.
I would suggest using FM to do what you do with XLS as Salesforce can be leveraged with FM
check the the MBS plugin - it has some functions for excel
Excel used to be able to link to FileMaker (earlier versions). Now it can use a text field (tab or csv) as a data source. Export from FM and use Data menu -> Get External Data. This allows you to "map" into an existing Excel Worksheet (even particular rows/columns)
You can also leverage the Excel APIs depending on your platform. Say it's Windows, then VBscript and PowerShell can talke to Excel and do the merge for you without needing ODBC. Those scripting languages are just plain text files so you can construct them and output them dynamically from inside FM and then have FM run them.