No, this is definitely not a stupid question
In fact TestNG offers the possibility to run JUnit tests! I've only tested this feature once and unfortunately it seems that it can only detect and run JUnit 3 tests (i.e. methods/classes with the necessary "test" prefix or suffix). Without more research I couldn't find a quick solution to run JUnit 4 tests. On the other hand the basic functionality is almost nearly the same in TestNG and JUnit 4. Both are heavily based on annotations and share a set of common features (for example @Test annotation for test methods). What's definitely annoying for developers coming from JUnit is the difference in assert statements. TestNG and JUnit mostly share the same assertXYZ() methods but unfortunately the order of expected and actual values is just inverse
In short: If you don't prefer a specific unit testing framework I'd recommend TestNG.
IDE and tools support is almost as good as for JUnit today. And TestNG offers almost the same features as JUnit plus some more which makes it for example more interesting to even create integration or acceptance tests. Multithreaded testing is possible (not only to speed up the test process) and some more.
On the other hand, if you have to deal with existing JUnit tests it's probabyl only a reasonable alternative if you a) can convert the JUnit tests or b) find a way to run JUnit tests from TestNG. Unfortunately I can't tell you more about it.
Marco