If an invention is meant to be a feature add to some other invention, which means it can't really function on its own, can this feature-add invention be patented?
Absolutely as long as it is useful new and not obvious. Examples include a carburetor for an engine, a windshield wiper, etc.
In another category would be an improved power lawnmower (maybe better gas efficiency) by someone other than the owner of a more basic patent on the whole mower. The inventor of the improvement - unless it is an add-on - can patent but can’t practice their invention since they can’t make the basic lawnmower. They can license it to the maker of the mower.