PowerPress does not use the get_field function, looks like a function from a 3rd party web site referred to as ACF.
Not using a 3rd party plugin, you can save your own info in the WordPress custom fields, either globally or by post. check out functions get_option and get_post_meta.
Just a word of caution, if you manipulate the RSS, you need to make sure it is still valid. Just adding HTML to it without any forethought could be disastrous.
