As per Aeroroute:
https://www.aeroroutes.com/eng/230802-a3oct23skpsjj
Aegean Airlines in October 2023 plans to launch Skopje – Sarajevo service, on board Olympic Air’s ATR72 aircraft. The Star Alliance carrier will operate this route 3 times weekly, effective 02OCT23.
Schedule below is effective 30OCT23.
A3986 SKP1915 – 2025SJJ AT7 135
A3987 SJJ2105 – 2205SKP AT7 135
Interesting choice. It looks like they will operate this as an add-on from the ATH-SKP flight, however interesting, not as a 5th freedom route. Flight numbers between the ATH-SKP and SKP-SJJ segments are different too. While they sell ATH-SKP, they do not sell ATH-SKP-SJJ at this time.