How to add an avatar and extra details to chat user?
Each user has a blob_id field http://quickblox.com/developers/Users#Response_3
Basically this is a link to file in Content Module. You can upload the file into the Content Module and connect it with user via blob_id field.
In case you would like to show additional user details, you can create a class in Custom Objects, e.g. UserExtraProfile, and write your user extra fields to this class. Other users can read these extra fields with help of “user_id” field as a filter for GET query.