Adding a new weapon model is pretty straight forward. You will need:
A new hak file. For this example, we'll call it ickyweapons.hak.
Inside ickyweapons.hak, you will need:
1. an updated baseitems.2da file that extends the "max range" of the desired weapon types to 255
2. a 3D model for each added weapon part (example: wblms_t_015.mdl - aka "weapon blugeoning morningstar top 1 color 5")
3. a 32-bit .tga icon for each added weapon part (example iwblms_t_015.tga)
4. the new texture/image file for the new 3D model
I've found that in order to get higher "colors" (5, 6, 7, 8, 9), you need to start using them with model number 1. After that, you can add higher colors for any other model number of the same weapon type. It's just that the game doesn't seem to recognize higher color numbers for other models until model 1's higher colors have been opened up / used first.