def xmlString = """<prnReq>
<ltrPrnReqs>
<ltrPrnReq>
<ltrData>encoded64 text</ltrData>
</ltrPrnReq>
</ltrPrnReqs>
</prnReq>"""
def xml = new XmlSlurper().parseText(xmlString)
def ltrData = xml.'**'.find{it.name() == 'ltrData'}
ltrData.replaceBody()
ltrData.appendNode {
Salutation('text')
}
println groovy.xml.XmlUtil.serialize(xml)
ZGVmIHhtbFN0cmluZyA9ICIiIjxwcm5SZXE+CiAgICA8bHRyUHJuUmVxcz4KICAgICAgICA8bHRyUHJuUmVxPgogICAgICAgICAgICA8bHRyRGF0YT5lbmNvZGVkNjQgdGV4dDwvbHRyRGF0YT4KICAgICAgICA8L2x0clByblJlcT4KICAgIDwvbHRyUHJuUmVxcz4KPC9wcm5SZXE+IiIiCgoKZGVmIHhtbCA9IG5ldyBYbWxTbHVycGVyKCkucGFyc2VUZXh0KHhtbFN0cmluZykKZGVmIGx0ckRhdGEgPSB4bWwuJyoqJy5maW5ke2l0Lm5hbWUoKSA9PSAnbHRyRGF0YSd9Cmx0ckRhdGEucmVwbGFjZUJvZHkoKQpsdHJEYXRhLmFwcGVuZE5vZGUgewogIFNhbHV0YXRpb24oJ3RleHQnKQp9CnByaW50bG4gZ3Jvb3Z5LnhtbC5YbWxVdGlsLnNlcmlhbGl6ZSh4bWwp