The EXST option to buy an extra free seat only works if you tag onto the entire trip.
https://www.sassalesinfo.com/travel-...eed-extra-seat
Hence the comment of the agent.
There should be an option for an extra seat in the NDC option of SAS (website or app) just like you can purchase upgrades for just one segment. However SPU is explicitly excluded:
Extra Seat will be available for all destinations except:
AYT, BIA, CAG, DBV, ESB, GZP, KGD, LCA, OLB, PSA, PUY, SJJ, SPU, SVQ, SZG, TFS, TIA and TIV.
https://www.sassalesinfo.com/travel-...ing/extra-seat