If I understand you correctly, I should assign a new group to the items I need, and then go through all the existing entities, find those that use the group that these items were previously in and add a new group to them.
This suits me, then I have a custom furnace that only uses coal and wood, but ...

I wanna create new furnace, it must use only coal and wood as fuel.
I created new fuel type - "light-fuel", and tried to add it for coal and wood...
But I ran into a problem fuel_category property of item it's a string, and it can't have more than one type of fuel
Do you have some solutions?