How do I append Google Test's XML output to an existing XML file?

So Google Test allows you to write output to an XML for logging purposes and possibly for consumption by a Continuous Integration service. I want to take advantage of this facility. Apart from the regular key-value pairs the default xml output provides, the RecordProperty("key",value) function allows a person to append addition user-defined info to the XML output. For example, one property would be (execution_time= 0.56ms). 

What I want to do is to get a time-series of these key-values, that I can later plot using a plugin in a CI to visualize the execution time taken by various tests after various changes. Essentially, I want google test to to append to the same XML file after running each test (each test is determined by its unique timestamp / commit ID / custom parameter). Does google test allow you to do this?

If googletest does not allow to append to an existing XML file, I am thinking of using rapidXML and replicate the same structure of a google test report (conforming to the JUnit format). However, can a JUnit-conformant XML-document consist of more than 1 ? Since each is a complete test run, can I have multiple testsuites in the same XML document and still conform to JUnit (so that Jenkins consumes it easily?)