Is there a way for Dorico to automatically add slurs over all notes that are occupied by a single syllable? Would save me a lot of time!
No, sorry.
I agree; this would be a nice feature to have.
Since I work in quite the reverse order, if the Dorico Team were to start adjusting how lyrics input and melismas work, I would rather have Dorico automatically advance the Lyric input line past slurred melismas; but frankly I am satisfied with the way this works now and think doing lyric placement and melismas manually may actually be just as convenient as not.
Not all slurs are melismatic, of course!

They are in my compositions. ![]()
But I realize some engravers need something different.
In fairness to @Derrek, this would be lovely for hymn work!

Any updates on this in Dorico 6?
It’s not in the Version History notes, no.
just searched for this potential feature myself. can we program “plugins” or “extensions” or maybe “macros?” does Dorico allow that?
There is a scripting language, but it can’t be used to accomplish this.