My question is: is there a document or other that explains how to represent the elements of uml diagram in universal xmi format ( for example in the class diagram how te represent a class, attribute.... in xmi format)?
Diagram definition and diagram interchange are separate specifications from the usual XMI. You can find them here: http://www.omg.org/spec/DD/1.0/ .
Diagram definition it is for the graphical representation I think!!
That's the only way to transfer diagrams between tools. If that's not what you asked for, please clarify your question.
I search the standard structure of XMI file representing uml diagrams (example :how to represent a class, attribute .... in an XMI file)
You cannot represent diagrams in normal XMI. You need to look at those links.