Cylinder AI bounds on objects are not supported. The ones that are still there are leftovers from imported DOS1 objects. I forgot why this is not allowed, but there's a good technical reason for it.
There's indeed no way to have multiple AI bounding boxes for a single object. The only way I know of to achieve what you want is to add additional invisible items with their own bounding boxes, e.g. as part of a prefab. Helper_Blocker is generally used in such situations.