Hello,
Yes, you can keep the footer content and add new contents. As you are using a template, instead of adding a footer that overwrites the existing footer, you need to set placeholders to the footer in the template and use this method to replace the placeholders by WordFragments:
https://www.phpdocx.com/api-documentation/templates/replace-variable-word-fragment-Word-document
setting the target as footer. Almost all template methods allow to set the target option, so you can replace placeholders by text, WordFragments, HTML and other contents.
Regards.