The Add Books menu has a sub-option "Add empty book (Book Entry with no formats)". So maybe create emtyp book records for your list of books to download.
https://manual.calibre-ebook.com/gui.html#actions
You can then find those entries by a search for "format:false"
https://manual.calibre-ebook.com/gui.html#search-sort
You could also then use the Reading List plugin and assign those books to a custom list.
https://www.mobileread.com/forums/sh...d.php?t=134856