You can easily revalidate an old collection on new data and merge newly generated strategies into it.

node .\bin\gen.js ^
  --settings               my-settings.ini ^
  --generate-then-validate            true ^
  --output-replace                    true ^
  --input  my-collection-[SYMBOL]-[PERIOD] ^
  --output my-collection-[SYMBOL]-[PERIOD]

How it works:
- generate-then-validate sets the Express Generator to continue generating new strategies after validation of the "input".
- output-replace - save the "output" over the existing file instead of adding a suffix. It will be helpful because you will not need to change the name at the next run.
- input and output point to the same file.

Additional:
It costs nothing to make it save the output at a particular interval adding: auto-save-at-minutes 5. It will leave the output over the same file every 5 minutes.