I think you may want to do it that you get the paths for the files and then loop over the list of paths, do your processing, create the new path for saving the result, save the result and then end the loop.
Prototype:
loopthroufiles.knwf (110.6 KB)
Overview
Comments:
- variable expressions is used to create a new path based on the old path
- Excel Reader/Writer use Flow Variables to define where files are read from and written to