When using the “Link popup menu” to define an external link, a list
appears that contains the external links that are already used in
 On a website with hundreds of external links, is there a way to
organize the “Link popup menu” (subdividing it into folders,
perhaps) that might reduce the amount of scrolling?
No. You can use the Hyperlink dialog rather than the popup, as that
doesn’t require you to hold your mouse finger down so much in order to
navigate the list of existing pages. The external links appear in the
Quick Links picker in that dialog, sorted alphabetically by URL.
 Is there a limit as to the number of external links the “Link
popup menu” can handle?
The only limit is your patience. As far as I know, there are no
(practically reachable) hard limits in this picking list.