Argos Train is designed to train each model from scratch on a single GPU in 24 hours and doesn’t support fine tuning. I also normally don’t save the model checkpoints after I quantize them and package them as a .argosmodel file.
If you want to train with custom data you should make your own argosdata file and then train from scratch. Since all of the existing data can be accessed automatically by Argos Train this should be pretty straightforward.
The 1.9 French model is actually an Opus-MT model and isn’t trained using Argos Train. So if you want to modify it you would have to find a way to modify their model and then re-convert it for Argos Translate.