Official comment
Unfortunately only the in/out points can be changed, but the transition time can not.
To edit the in/out points, you'll have to adjust the phrase placement on your track for the INTRO and OUTRO segments. It's a bit complicated to explain in text, this video (while in French) explains the process of editing the phrases and how that affects the automix transition points.
We hope to include additional controls for these functions in a future update.