Simple:
1. Save a copy of the workbook with the name 'DataSource.xlsm';
2. Set up a mailmerge whose datasource is your 'DataSource.xlsm' file;
3. Whenever you want to run a mailmerge, save a copy of the workbook as 'DataSource.xlsm', then run the mailmerge.
That way, Word need never be concerned with changing datasources - for all it 'knows', 'DataSource.xlsm' is the same file it's always worked with.

You talk of doing this with 'one click of a button'. For that, you could have a button in each of the Excel workbooks to call a common macro that saves the copy then runs the mailmerge. The user need never touch Word for the process.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]