Hi Victor
Sure, if you have your own external scenarios than you need to implement a ScenarioGenerator that loads them from your file and returns complete Scenarios when requested.
https://www.opensourcerisk.org/docs/orea/classore_1_1analytics_1_1_scenario_generator.html
The ScenarioWriter is only used if you want to examine the generated Scenarios in Excel or R or something like that.
Regards,
Niall.