Unfortunately, it is not that easy. You’d have to merge these GD / RoT files:
- database.arz
- Items.arc
And then copy the following from ReignOfTerror/resources into Grim Dawn/resources:
- items_rot.arc
- ith.arc
- np_items.arc
- textures.arc
- textures_rot.arc
- Xpack.arc
- zakks_d2items.arc
There are guides on how to extract & compile .arz / .arc files in the “Modding - Tutorials and Help” section.