Hi mpb2000,
Unfortunately, the Write to Measurement File ExpressVI does enable you to write units on the Y axis label. The label on the Y coIumn is determined by the name of the signals coming into the VI. I suggest you use lower level File I/O VI's which give you more control over the formatting of the output file. If you search for "Spreadsheet File" in the LabVIEW Example Finder (Help » Find Examples...) you can find a few examples that can point you in the right direction.
I am not 100% sure this is what you were asking so please let me know if this answered you question.
Eli S.
National Instruments
Applications Engineer