Extension of patent term

It is possible in theory, but it depends on the protection scope of the original patent. If the protection scope of the original patent is relatively large, then the new application within this protection scope cannot pass the examination and authorization, and some minor improvements may be covered by the previous patent and cannot be authorized. Only by further limiting the technical characteristics of the original patent and narrowing the scope of protection can we obtain authorization. But in this way, you can't protect your technical content well. So how to establish a patent pool is a very difficult thing.