What did someone else invent, and then I added more functions to his invention, so that I could apply for a patent myself?

There is definitely no problem with the application, but it is hard to say whether it can be authorized.

This needs specific analysis. If it is a simple superposition of existing functions, it does not meet the creative requirements and cannot be authorized.

If it meets the requirements of novelty (not existing technology, no conflicting application) and creativity, it can be authorized.

Furthermore, you applied for and authorized it, but do other people's things also have patent rights? If so, it depends on whether your design belongs to his patent protection. Otherwise, even if you have a patent, you must get his authorization to implement your own patent.