Steffen
The capabilities xml file for WMTS is automatically generated by ArcGIS Server using a jsp process and I would not recommend changing it except in extremis, with a backup and accepting the risk. What you can do if the WMTS service is stable and not changing is to copy the WMTSCapabilities.xml file, edit it to add say contact details and then publish it through a web server, such as IIS as long as the http internals listed in the WMTSCapabilities point back to the original ArcGIS Server.
Yours aye
Jim Sibbald