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Not too hard!

Just download the free SafariSort app. Makes the sorting process for Safari bookmarks quick and painless.
Yes, I know, not really the same as "adding an option" - but if you searched around for how to do that, you would notice very quickly that Safari does not make that process as simple as most other browsers.
Can't say why.... But the SafariSort app does make it easy enough that you can change your sorting as often as you like.
 
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