Python xml

app.py

import xml.etree.ElementTree as ET


def indent(elem, level=0):
    i = "\n" + level*"  "
    if len(elem):
        if not elem.text or not elem.text.strip():
            elem.text = i + "  "
        if not elem.tail or not elem.tail.strip():
            elem.tail = i
        for elem in elem:
            indent(elem, level+1)
        if not elem.tail or not elem.tail.strip():
            elem.tail = i
    else:
        if level and (not elem.tail or not elem.tail.strip()):
            elem.tail = i


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

for person in root.findall('person'):
    full_name_el = ET.Element('fullName')

    first_name = person.find('name').text
    last_name = person.find('lastName').text
    full_name = f'{first_name} {last_name}'

    full_name_el.text = full_name
    person.insert(0, full_name_el)

indent(root)
tree.write('output.xml', encoding='utf-8', xml_declaration=True)

input.xml

<?xml version='1.0' encoding='utf-8'?>
<data>
  <person>
    <name>1ÅÅ</name>
    <lastName>1ÄÄÖÖ</lastName>
  </person>
  <person>
    <name>2BB</name>
    <lastName>2YYYY</lastName>
  </person>
</data>

output.xml

<?xml version='1.0' encoding='utf-8'?>
<data>
  <person>
    <fullName>1ÅÅ 1ÄÄÖÖ</fullName>
    <name>1ÅÅ</name>
    <lastName>1ÄÄÖÖ</lastName>
  </person>
  <person>
    <fullName>2BB 2YYYY</fullName>
    <name>2BB</name>
    <lastName>2YYYY</lastName>
  </person>
</data>