How do I add new artifacts?

well, in mod 5.5, naf is also used, and it seems like everything is generated correctly there. There's also a map editor there; you can take a look.

Added 38 minutes later

So, when a new map starts, will new artifacts appear in place of the random artifact boxes? And if the GSC is making a map, will they not appear there either?

That's a bit disappointing. That's not what I was aiming for.

In general, try creating a map in the naf editor first and, in the list of artifacts in the map settings, check all the new ones (the editor should do this automatically) and see how they will be generated.