In batch processing mode you can set a wildcard for worksheet names and then append them all to a single output file. Example 2 here shows how to do it for multiple CSV files:
Change from *.csv to Mysheet.xlsx[*] to do the same for sheets of a spreadsheet.
So that will automatically Stack all the sheets in a new file (perhaps single CSV). You will then have to process the new dataset in a second .transform.
Your other option is to drag the spreadsheet on to Easy Data Transform. Select only the new sheet and then drag a connection from the new sheet to an existing Stack transform.
It would be better if you could put a wildcard in the input file path (file name and/or sheet name) and it would stack all matching files/sheets for you automatically. That is on the wishlist.