Hello,
Sorry but that feature is not available for phpdocx 7.0, only for phpdocx 7.5 (and newer when new releases of phpdocx appear).
If you have the LUS (https://www.phpdocx.com/updates) for your license you can download the latest package on MY PHPDOCX after login. Otherwise, you can upgrade your license on this same page. We have tested the previous script with phpdocx 7.5 and all merge field styles and properties keep.
Regards.