From this GeoServer tutorial, (under "Creating a store") I can only see Vector, Database and Raster data types as the only possible data sources. But I was wondering if a Web Mapping service such as a WMTS could be added as a source?
No, it could be added with some programming work, but right now GeoServer can only cascade WMS.
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2With GeoServer 2.12.x it will be possible: docs.geoserver.org/latest/en/user/data/cascaded/wmts.html – Saleika Sep 13 '17 at 9:34
Since version 6.2, Mapserver raster input support is through the GDAL raster library only.
And one of GDAL's supported formats is ..WMTS. – elrobis Mar 18 '16 at 19:16