For Dutch (swamp German ;p ) “uitbreiding” (similar to Erweiterung") felt the most natural. But I can only guess by name what an AddOn is. I’d guess some kind of paid, extra service tied to your Jolla account, perhaps related to the AI direction Jolla is heading towards?
Semi-offtopic: Firefox uses Add-on in Dutch, but I don’t agree with them. I’m in favour of sticking to Dutch words when it sounds natural. The everyday Dutch language is already showing enough signs of English due to some translators in the technology field being lazy.