Just an update since i recently had a flight from Europe to Kenya via ADD.

Exit rows are given at checkin as Goaguy mentions. Something i realized is that they can be accessed only by the station you are checking in, meaning that if you checkin e.g. in FCO all the way to CPT, then they can give you exit row seats only for the FCO-ADD, not for the FCO-CPT.

I saw on expertflyer that the seat next to be was blocked 3 days before departure. However, on the Europe-ADD leg, i chose an exit row and then the ADD-Kenya leg was full. On the way back, I got exit row on Kenya-ADD leg with the seat next to me empty. For the ADD-Europe leg, i had 2 people next to me, although there were plenty of middle seats towards the back empty.