It would be nice to, when setting the option to split a category by expansion (in my specific use case) to include items that were manually assigned to the category in the split. This is especially helpful with crafting materials, such as Fiery Cores being considered general reagents as opposed to elemental regeants. I'm using custom categories so I can split materials by both type and expansion, and it leaves a few stragglers and/or custom types isolated.

This feature exists now.