To do what you want is like this:
Regardless of the name of the workbook, the MS Excel VBO action "Attach" will attach to any instance of Excel that is currently open. The "handle" data item will be output, which you may utilize with the "Get Worksheet as Collection" action.
In this manner, there is no need to save the file. However, keep in mind that if you are running multiple instances of Excel, this will not work.