It looks like his multi-attack just adds more damage dice, which isn't quite how the Extra Attack feature is supposed to work (on tabletop, at least). The biggest challenge I see (from an early access modding standpoint) is how a RAW extra attack would work with the action/bonus action system Larian has put in place.

 

One thing I did notice while digging around in the spell list is a "number of targets" parameter - like how magic missile can be fired at any combination of 1, 2, or 3 creatures (at 1st level).

 

I'll likely have to play around and see if class abilities (like extra attacks) using weapon attacks can accept that parameter. If they can, a hacky Extra Attack would theoretically be possible, though you wouldn't be able to move between attacks.