The LVM Express VI does not have a way to change the header, since it was designed for speed of implementation and ease-of-use, not flexibility. However, you can change it to a normal VI and modify the existing code (make sure you save all VIs to a new location so you do not overwrite the vi.lib versions). Right-click on the Express VI and select Open Front Panel. Be warned - this is a complex VI, since it handles a lot of options. You will need to open several layers of VIs to get to the LVM write section. There is a single VI which writes the header that you can modify.
Another option is to use the subVIs used by the Express VI to create your own VI. Be aware that using unpublished VIs in this fashion can result in broken code when you upgrade to newer versions of LabVIEW. These are unpublished and unsupported interfaces and can change, if needed, at every LabVIEW version. These particular interfaces tend to change about every two or three LabVIEW versions.
It may be easier to write your own header, then use the Write to Spreadsheet File VI to write your data.
