Is there any way to add custom field keys to the dropdown of the custom fields on the "add new post" page without adding the fields?
Check add_meta_box
(It allows plugin developers to add sections to the Write Post, Write Page, and Write Link editing pages. ):
http://codex.wordpress.org/Function_Reference/add_meta_box
Or these plugins:
Source: http://sltaylor.co.uk/blog/control-your-own-wordpress-custom-fields/