As noted above, when generating code for EXTENDED-XER, you will be working with the "XML" runtime, as oppposed to the "XER" runtime. This makes coding for EXTENDED-XER slightly different from coding for XER:
You will use Asn1XmlEncoder instead of Asn1XerEncoder
You will supply the name of the element when encoding a value. Typically, this will be the name of the ASN.1 PDU type.
Finally, there is a sample reader and writer program in java/sample_xer/EmployeeEXER, should you need to see an example.