If you are only planning to deploy the master process, then you will need to extend the properties (that you defined the sub process) in the master process that will be deployed. This will allow you to see the extensions in the extensions tab.
Note the following part
or any child process
of this line:
Clicking the Extensions button above the process canvas opens the Extensions dialog for the purpose of defining extensions for components referenced in the process or any child process.
in this article that explains how extensions can be defined in a process: