Ok, the ODBC driver will need to know where to access the excel file and it can't be supplied with a Laserfiche login to go get it :)

If you go to your administration console and configure a default repository under server settings then you should be able to access all files in that repository by the following path \\lfservername\lf. Test it out in Windows Explorer. You will notice none of the files have extensions, but if you open the excel file in excel, it will open as a native excel file.

If you only have one workflow instance running at a time you could copy the excel file to a temporary folder with a specific name that the ODBC connection is expecting.

Otherwise you may need to programmatically configure your ODBC connection strings.

Edit: LF handles sharing based on Access Rights for Windows Authenticated users but this could cause other security vulnerabilities as tags and scope do not exist in Windows. The more secure method may be to temporarily export the excel file to a hard drive.