Community articleIBM Connections extensions are not supported through ATOM API
Added by IBM contributorIBM on September 5, 2012
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No abstract provided.
IBM Connections extensions to the Activity Stream spec, such as "connections: {rollupid: 3d7638dd-b5e1-4d52-a03d-83591996da20}", are only returned and handled when using the JSON API.

In ATOM/XML API, it is not supported.

According to:

I recommend using the JSON API when handling extensions.  The JSON API fully supports all CRUD operations with extensions "out-of-the-box" by simply extending ExtendableBean.  XML serialization requires a couple of extra steps and has a number of limitations due to Shindig's design and use of xstream. 
  • To serialize extensions to XML, the extension's field name must be predefined in the parent POJO.  For example, to properly serialize the 'extensions' bucket in the Introduction example, the 'extension' property must be hardcoded in (and the implementing class).  Fortunately, once the property is added, nested extensions within the bucket are supported to an arbitrary depth.
  • Very limited XML POST support.  This has been a longstanding limitation of Shindig.  Only the most basic XML POST requests succeed (only string key/value pairs, no Lists or Maps).
  • Serialization of extensions do not follow all OpenSocial conventions.  The following illustrates the serialization of a JSON extension which includes simple string key/value pairs, a list of strings, and a list of objects:...
To support "extensions" in ATOM/XML api, it is necessary to modify the ActivityEntry implementation.

And this is only for GET API, "extension" is not supported in XML POST. This is a known limitation of shindig, and there no workarounds on it currently.