Hi Rogerio,
Net Express 3.1 does not provide for any internal method of generating XML files.
We first added the XML-IO preprocessor allowing you to directly read and write XML files starting with NX 4,0 and we first added support for the XML GENERATE statement starting with NX 5.0.
I would highly recommend that you upgrade to either the latest version of NX which is 5.1 or to its successor product Visual COBOL. Visual COBOL, in addition to the two methods I just mentioned also supports XML extensions which is the method introduced in RM COBOL.
Thanks.