After going through documentations and quick-start tutorials available for Google cloud Endpoint it feels like it currently supports json, where as Google Cloud Storage has support for both json and XML. I wanted to know if Google endpoint supports XML? in case I missed it in the ref documentation.
Google Cloud Endpoint service definition is based on Open API v2 version, also know as swagger 2.
This specification is only in yaml format and you don't have the capability to use XML for this definition.
Anyway, today, there is a lot of libraries/frameworks that implement the Open API specification to generate automatically the API spec from your code, or to generate code stub from an existing API spec. It's one of the most popular standard today.
EDIT
According with your comment, if you want to add a body in XML, is up to you. It's out of Cloud Endpoint scope, where are on pure HTTP protocol here.
If your XML is too long, too complex because you have double quote and simple quote (and you have to escape them correctly,...), you can do this
body.xml--data @body.xml