I'd just like to add that I've now found a workaround for the thing I wanted to use add_to_extracategories for.
So I don't need it to work
In case this is a larger problem though: for some reason add_to_extracategories still doesn't work, even if I put it in overrides. That might be to do with custom columns, or something in my replace_metadata, I don't know.
But I've done what I set out to do, so I'm happy. Thanks again for the prompt response and have a great day