I am not sure that I follow what you are trying to do, but Excel files lock when you open them, so only 1 user can have it open at a time.  Also, workflow is a service, so it has no desktop and cannot interact with the desktops of (or applications running by) other users.

That said, you can use ODBC to push data into an Excel file or you can use a Script activity and reference a DLL to interact with the file directly.  See Calling excel's vba subroutine from vb.net application for an example of using VB.net to execute vba in Excel.