Is it possible for you to add a JSR-223 step to perform the action? Haven't tried this in the latest versions of DevTest.
Assume that a prior step created a property called "flatXML" and that contains your XML.
import com.itko.lisa.remote.utils.Base64;
message = testExec.getStateValue("flatXML");
String encodedMessage = (String) Base64.encode( message.getBytes() );
testExec.setStateValue("encodedMsg", encodedMessage);
return encodedMessage;
In a different step, decode the encoded message
import com.itko.lisa.remote.utils.Base64;
encodedMessage = (String) testExec.getStateValue("endodedMsg");
byte[] decodedMessage = Base64.decode( encodedMessage );
String msg = new String( decodedMessage );
testExec.setStateValue("decodedMsg", msg);
return msg;