Generally speaking, if you want to add a new context you would use the *_custom.conf file. If you want to modify an existing context you add lines to the corresponding *_custom_post.conf file. For example, if I wanted to modify the language for PJSIP extension 2004 (without using the gui), I would add the following lines to pjsip.endpoint_custom_post.conf:
[2004](+)
language=es