Export, edit, import changes

I've spent some time readying through and searching this forum, but am having difficulty finding the info I'm looking for.

As I've added video content to my library over the last few years, I've added a title description (%SUBTITLE%) to each file. I am wanting to copy the individual descriptions into a new field as well (%PODCASTDESC%). I've aligned the two field columns next to each other and have been copying/pasting, but with over 1,000 titles left to go, I started to look at export/import options.

There is an export to .csv that looks intriguing. It does not contain those two fields by default, though, and I haven't been able to find how to edit those export fields ("Export Configuration Archive" didn't seem to help).

Then, once I've edited each, I didn't see how to import the changes?

Perhaps this is not possible using Mp3tag? Or, there's another function that can complete what I'm attempting? Thanks for any helpful information.

Convert "Tag - Tag"
... or ...
Action "Format value"
Formatstring: %SUBTITLE%