Okay, i got told how it works to add a material. First be in the main category. Filter must be off. Then create a material. Save the blend file. Add the currently selected material with the Add button. The material should be in the list now. Select it. Choose the category where you want it to be. Then press the “Add, Remove. Reload Apply …” button. The one with the back arrow icon. And finally tick the filter button. The material should be in the category now. And is stored in the library.
Phew 