Hello,
Using modifyPageLayout (https://www.phpdocx.com/api-documentation/layout-and-general/modify-Word-page-layout-with-PHP) you can set custom values: width, height, marginTop... you need to apply custom as paperType. Other paper sizes are preset values of these options. There's no wa to inject new paper sizes as preset values, you need to use the available custom options.
Regards.