Thanks for the comments.
I will try to use MrC's suggestion from the thread mentioned by jmone (
https://yabb.jriver.com/interact/index.php?topic=87939.0):
1) Create a mpl playlist from some movies that already are in JRiver.
This way I get a type of file JRiver can re-import.
2) Modify this playlist by copy-pastes from my existing spreadsheet.
(Perhaps then I should export a mpl playlist with the number of entries I want.)
3) Then import the final mpl playlist.
MrC writes you don't always need to have dummy files.
But I will create dummy ifo and bdmv files and folders.
[ifo and bdmv files are just very small files, so having them in a special folder on one's computer is really not a problem.]
This way, when I update the data about a movie within JRiver, it can export a sidecar xml file with all the data about it, and more important, it can also store the picture of the movie, which I want.
Once I have my data on JRiver, I will only modify that database (the Library), not my original spreadsheet.
And I can use JRiver to export all sorts of spreadsheet that I can print on paper.