Is it possible to create a script which runs after the source is loaded but before the import.
This will be a hack in RO CSVI core and not possible with custom plugins. I would recommend to write independent custom script to read the CSV and convert into needed format and save as new CSV file before running the import. You can write it as a bash script where each command is a line on its own. So the first line will be your custom script and saves the new file on your server. RO CSVI price import runs next and reads the file from the location saved on your server. You can setup a cron job to execute the bash script.
I know there are rule plugins but I think it can't be done with the existing ones.
Yes existing rule plugins do not support this feature.
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