Add custom Yoast variables. Code example

function register_custom_yoast_variables() {
	wpseo_register_var_replacement( '%%kilkist%%', 'get_kilkist', 'advanced', 'some help text' );
	wpseo_register_var_replacement( '%%myname%%', 'get_myname', 'advanced', 'some help text' );
}

add_action( 'wpseo_register_extra_replacements', 'register_custom_yoast_variables' );