Calibre has a plugin, "Hyphenate This!", that will insert soft hyphens into a document using a Libre Office foundation hyphenation dictionary. It would be very nice to have a similar capability in Sigil.
I don't know if the two plugin approaches are similar enough for this to be helpful, but in case here is the thread about the Calibre plugin:
https://www.mobileread.com/forums/sho...d.php?t=208534
Dave