Sorry I'm not a real video enthusiast, so if nothing else I'll bump you post
I'm guessing that the Description field in JRiver is the Kodi plot field. You can remap field names to custom fields provided that the metadata is written to the file. Thats normally not a big issue with audio files, but you are talking about a video file. Most video "tags" are contained in sidecar files (ending in xml extension), depending on the format. I have no clue about Kodi's tagging. But check to see what is written to the file you are importing ... if "plot" exists (sometimes what you see on the screen is not exactly the field name in the file itself), create a custom field
exactly as it shows in the metadata of the file. If it its in a sidecar file, open the xml file and check.
I have never tried this but maybe if the data in contained in an xml file it could be imported from that ... really not sure. Again to test you'd have to create a custom field using exactly the same name. It doesn't hurt to try. If you can import your field in the new custom field, you could then easily port it over to the standard "Description" field, just by selecting all the files and adding a little expression.
Short of this you could have JRiver retag this information for you, which is semi-automatic.
Mmaybe someone else could chime in to give you more definite information ?