I am using "export to excel" smart service to load the process variabl

or is it possible to merge into a single file after generate separate excel spreadsheets?

You might need to do the following:

1. Populate the first sheet in the smart service
2. Add a second smart service to populate the rest of the sheets one at a time (using MNI for example with the option, launch instances one at a time) but making sure that "Document_to_Overwrite" points to the output of the first node from step #1.

i am using the sub-process to populate sheets starting from initial sheet, and passing the same output document to the rest of the sub-process, still I am facing the same issue.

Any workaround?

Are you seeing a new version of the document gets created every time? You should be able to see version numbers under /suite/designer -> Documents

I'm asking this because the code looks good to me. I am not the author but it seems that if you configure it correctly it'll create a new version of the document every time.

Are you sure you're running the sub-process one at a time?

yes.