Same as a _chr.txt file for a specific zone:
1) Open /Resources/GlobalLoad_chr.txt in a decent text editor --
not plain old Notepad, needs to be something that respects standard line endings (pretty much anything else).
2) Check the list at
http://wiki.eqemulator.org/p?NPC_Models&frm=Main to find the 3-letter model code and source file for the model you want to be loaded.
3) Add a new line and type in the model code and source file in the format <model code>,<source file> -- for example liz,oot_chr or rtn,rtn. If you add your new line at the end of the file, make sure there is one empty line after it.
4) Make sure the number in the first line of the file matches the number of models being loaded (i.e., increase it by 1).
5) Make sure to close your client completely before testing so it will be forced to load everything from scratch.