The fundamental issue here is that Dorico simply will never show an extender line for a lyric that has no duration. Lyrics only get a duration if you hit Space during input to advance past another rhythmic position in the voice to which the lyrics belong. At the moment there’s no way to edit a lyric to give it a duration short of making sure there are notes at subsequent rhythmic positions in the voice to which the lyric belongs, then open the Shift+L popover and hit Space to advance it.
When we have a bit of time to think about this issue further (which is not now, because lyrics are not the focus of the work we’re currently doing) we’ll see whether it’s feasible for us to add some other kind of editing operation that will make it possible to change the duration of a lyric event without there being notes in the voice to grab hold of.
In the meantime, I suggest that the practical solution may be to simply add another couple of notes in the Soprano voice temporarily in order to get the lyric duration to be extended, then remove those notes again.