Thanks for your question!
Unfortunately, there is not a way to generate only a load file from a previous production. However, I believe you can achieve what you're looking for by creating a copy of the target export, removing the settings to overwrite the production numbering, incrementing the volume number, and exporting the production numbers that are currently assigned.
You would still need to "re-do" the export, but creating a copy would allow you to make the necessary load file changes without affecting the original production.
Keep in mind we are always looking to improve our products. If you'd like to see an option to generate load files based on previous productions I encourage you to Submit a Feature Request directly to our product team.