import xml.etree.ElementTree as ET

tree = ET.parse('feed.xml')
root = tree.getroot()

for elem in root.iter():
    if elem.tag in ['info', 'organization', 'group', 'team']:
        ET.SubElement(elem, 'formal_name').text = elem.find('name').text if elem.find('name') is not None else 'Formal Name'
        ET.SubElement(elem, 'formal-name').text = elem.find('name').text if elem.find('name') is not None else 'Formal Name'

tree.write('feed2.xml')