Robot Framework does have tool called rebot
which lets you create report from multiple output.xml files, but I think that TestNG provides only xUnit like XML file which is not the same so rebot
is no good here.
Depending on how many tests you currently have in TestNG, I would either let Robot Framework launch the TestNG tests, parse the result file and mark a test passed if there are no fails in the result file. Or let the RF tests assume that the result file from TestNG run is available in certain place before RF tests start, and then create one light test for each TestNG test. Each of those tests would only check from the TestNG result file did the corresponding test pass or not, and then pass or fail the RF test.