Все нормально добавляется. Разбирайся.
>>> import xml.etree.ElementTree as ET
>>>
>>> ET._namespace_map
{'http://www.w3.org/XML/1998/namespace': 'xml', 'http://www.w3.org/1999/xhtml': 'html', 'http://www.w3.org/1999/02/22-rdf-syntax-ns#': 'rdf', 'http://schemas.xmlsoap.org/wsdl/': 'wsdl', 'http://www.w3.org/2001/XMLSchema': 'xs', 'http://www.w3.org/2001/XMLSchema-instance': 'xsi', 'http://purl.org/dc/elements/1.1/': 'dc'}
>>> ET.register_namespace("CadEng4", "urn://x-artefacts-rosreestr-ru/commons/complex-types/cadastral-engineer/4.1.1")
>>>
>>> ET._namespace_map
{'http://www.w3.org/XML/1998/namespace': 'xml', 'http://www.w3.org/1999/xhtml': 'html', 'http://www.w3.org/1999/02/22-rdf-syntax-ns#': 'rdf', 'http://schemas.xmlsoap.org/wsdl/': 'wsdl', 'http://www.w3.org/2001/XMLSchema': 'xs', 'http://www.w3.org/2001/XMLSchema-instance': 'xsi', 'http://purl.org/dc/elements/1.1/': 'dc', 'urn://x-artefacts-rosreestr-ru/commons/complex-types/cadastral-engineer/4.1.1': 'CadEng4'}