Force specific folder to be added to Photos?
Delete the .nomedia file to force media scanning  ( 
restart the phone. It is then that media scanner database is re indexed )
and it should appear in Google photos folder list for sync.
If you don't want that folder to be scanned by media folder at all, create a new folder in gallery ( use a third party file explorer from Play store , if your built in explorer can't do this ) and copy those photos into this folder manually. Verify, it's showing up in Gallery options to sync
Obviously, you wouldn't want to manually do this every time you use that app, so automate it. Macrodroid app implement it this way


Trigger: Application Launched / Closed &rarr; Closed &rarr; Select your app

Action:  File operation &rarr; Select   paths and move / copy options

This ensures that everytime you close your app, pictures are moved / copied into gallery sub folder and available for sync