This can be done with activeX. NI has some examples of writing to excel using activeX. In them you'll see where they open a worksheet by passing in the name of the worksheet, you can open multiple worksheets using this method.
Try this link
http://zone.ni.com/devzone/devzoneweb.nsf/opendoc?openagent&44A46A59D48763FD8625683A000AAFC4&cat=9C6DF90777E5A78206256874000FA14E
Brian