Set a value to an attribute in property transfer

Sure. you should use "//" in root node.
if you would like to use groovy script to set an attribute in custom property. that's no problem to do that. here is a simple example:

#1:(need to create a script step. note: the script cannot be run directly, if you define testrunner in test suite or test case level, you should run it in test suite or test case respectively)

import com.eviware.soapui.support.XmlHolder
def ts = context.getProperty("#TestSuiteRunner#").currentTestCase.testSuite
def xmlHolder = new XmlHolder(context.expand ('${StepName#Response}'))  //populate your test name
xmlHolder.namespaces['ns1'] = 'http://.............................' 
def node = xmlHolder.getNodeValue("XPath expression")            //just like you used in xpath assertion
ts.setPropertyVaule("node")    //set the value in custom properties of test suite level
 

Or

#2:

def groovyUtils = new com.eviware.soapui.support.GroovyUtils(context)
def ts = testRunner.testCase.testSuite
def xmlHolder = groovyUtils.getXmlHolder ("StepName#Response")    //populate your step name
xmlHolder.namespaces['ns1'] = 'http://.............................' 
def node = xmlHolder.getNodeValue("XPath expression")            //just like you used in xpath assertion
ts.setPropertyVaule("node")    //set the value in custom properties of test suite level

thanks,
Aaron