I have Chinese Z visa in a different passport and now in China with a L visa.

yes, you need a Hong Kong visa to travel there. Now, I want to make clear one thing: You are in China with a L visa now, how do you get Chinese Z visa in a new passport? It is illegal to have two passports at the same time. Once your new passport is issued, you old passport expires automatically.

Now, you can exit Mainland China with your L visa and go to Ghana, then there are exit and entry stamps on your old passport, you can use Z visa on your new passport to enter China.