Yes, you can.
In my experience of AONE3,
I bought two extra segments.
(In total, I have 3 return trips of HKG-NRT)
Some of NRT-HKG segment have first class seats.
I selected a business class flight in mid itinerary such as
HKG-NRT A
NRT-HKG D
HKG-NRT A
NRT-HKG A
HKG-NRT D
NRT-HKG A