Sam, in 2000 Weaver offered 2-car add-on dome sets for their NP and GN five car sets produced the year before. I don't recall the catalog number for the NP dome set but for the GN dome set it was G-1612-L (for the three rail version) so the number for the NP set should be something close to that. (I did have the NP set but sold it in 2003. I still have the GN set.) I think the MSRP was around $240 at the time.
I hope this helps. Jim Rawlings here on the forum bought and sold these cars a few years ago and he might recall more details if you contact him. Good Luck!