Office extensions supports the ability to inject xml parts from files in the content folder as a custom xml part in an active document.
<Cmd case="BaseExtensions:CreateXmlPart" path="x" override="x" />
Required. Defines the path to the xml part to be injected. The root folder is assumed to be Content/BaseExtensions. The xml part must be saved as an xml file in this folder and the file extension should be included in the path definition. Example
Required. Defines whether or not the xml part should overwrite an existing part with the same name if one is found in the active document.
NOTE: It is entirely possible to have multiple different files with the same part name within the content folder but we can only inject one version of an xml part. See HitachiEnergy for an example of how this can be implemented.