How to transfer/add a property to an array in the next soapcall

I'm not sure if this is the best way, but I have done what you are trying to do with a Groovy TestStep using the Groovy XmlParser, though this was slightly complicated by the fact that the XML has namespaces.

I've tried to match the code to your example, so it's not tested and might need to be tweaked a bit (I've also assumed that Request body will be wrapped with an envelope element, again if not adjust as needed).

import groovy.xml.*
import groovy.util.XmlParser
import com.eviware.soapui.support.GroovyUtils

def request = context.expand( 'Reference To Your Soap Request' )

// Define required namespaces as our paser will be namespace aware
def soapenv = new Namespace('http://schemas.xmlsoap.org/soap/envelope/', 'soapenv') 
def ns = new Namespace('http://yourfirstnamespace', 'ns') 
def arr = new Namespace('http://yoursecondnamespace', 'arr')

// Parse the namespace aware XML SOAP Request
def parser = new XmlParser(false, true)
def envelope = parser.parseText(request)

// Obtain the node we want to append too. 
def resultIds = envelope[soapenv.Body][0][ns.ResultIds][0]

// Create and Append the new node
def guidQName new QName('http://yournamespace', 'guid', 'arr')
resultIds.appendNode(guidQName, 'GUID Value here')

// Update actual test step with new request data
def groovyUtils = new GroovyUtils(context)
groovyUtils.setPropertyValue('SOAP Test Staep Name', 'Request', XmlUtil.serialize(envelope)) 

Here are the links for the JavaDocs for the XmlParser, Node, and QName objects.