Do you have the Report Generation Toolkit? If so, the Excel Easy Graph VI has an input for that.
If you don't have the toolkit then you will need to use ActiveX. Please do a search on how to use ActiveX (there are examples that ship with LabVIEW) to control Excel. There are also lots of example in the Excel thread. NOTE: DO NOT POST QUESTIONS IN THAT THREAD.
Ultimately you will need to look up the information in MSDN.