Downloaded a decompiler and had a look at the dll, found where the default item categories are created.
I believe for the next step I'm going to learn how to run my own code and see if I can add a custom category.
Just for reference if someone in the future searches for something related here is where the default item categories are created.
Item category
...\Mount & Blade II Bannerlord\bin\Win64_Shipping_Client\TaleWorlds.Core.dll
TaleWorlds.Core > DefaultItemCategories.cs