easiest way is do it on a spare install.

You need alpaca's version of the items.txt file for the old entries so the entries get into the db file. https://forums.totalwar.org/vb/showthread.php?t=60818

add your new entries to the top of that file and put your models in right folders.

delete / rename the items db file

start game - when error messages come up click through them until you get to buildings you know are below your new entries in the list

copy the db file and the new items into your actual working install



reason for doing it on 2nd version is if you click through too many entries it will start making blank 1kb items for the non-existent cas files replacing the original items (so make sure you don't end up copying those into working version)