Hi, all
I’m looking more closely, these days, at the arranger track and maybe I am missing something that seems obvious to me, but it doesn’t seem that we are able to define directly an arranger event from a musical container (MIDI part / audio event or part) selection.
A drag & drop of the latter to the existing and active arranger track would define/add automatically an event which would appear as so in the arranger editor, this with its boundaries set where the musical container has been dropped. IMO, It would speed up considerably the composition of a song, combined with the already functional Snap to event feature.
But again, maybe something went off my radar… 
(Cubase Pro 15.0.21)