Are you in the process of developing this application? If so, then the best option for you is to use the full PI SDK as it is accessible via Excel VBA, and includes the functionality you're looking for -- with lots of examples. Those examples can be found here on vCampus, and in the PI SDK help file (installed with the PI SDK to %PIHOME%\help\pisdk.chm).
Otherwise, if you've got existing code that uses PIPutValX, we can look at options for doing what you want - but the PI SDK is better documented and more full-featured.
Are you referring to Result = Application.Run("PIPutValx", sTagname, valueCell, stime, ServerName, resultCell) where you can pass the PI Server where you want to write to?
Anna, PI SDK or AF SDK is more powerful but for one off tasks, PIPutValx is good enough.