I've never really thought about this before, I don't think the books really give any idea about weather the other islands where part of the Narnia kingdom or not, though it has been a few years since I last read the books so it's possible I'm forgetting something.

Personally I've think Galma, Terebinthia, and Seven Isles where their own separate country's. It's certainly possible that one or all of them were part of Narnia like the Lone Islands, but I don't think it would make sense for Narnia to have a bunch of territories, Narnia doesn't seem like the kind of country that would be interested in colonization. I always got the sense that the Lone Islands were an exception since they willingly became part of Narnia in gratitude for being saved from a dragon. With Terebinthia and the other islands I imagine they came to be the same way Archenland did, with humans from Narnia going of and forming their own countrys.