CRTX files are chart templates were introduced in one of the previous versions of Microsoft Office, but they can still be used with the software to customize various settings and elements to apply the same style across multiple charts.
This means that crtx to doc conversion is more about using the template to change charts inside existing document, than to be a direct file conversion. But in a way this is doable, just by manually applying the template. These templates are typically stored in the Charts folder.
Updated: October 27, 2023