You can easily convert JUnit 3 and JUnit 4 tests to TestNG.
Your first option is to use the Quick Fix function:
Convert from JUnit 3
Convert from JUnit 4
You can also convert packages or entire source folders with the conversion refactoring:
The refactoring wizard contains several pages:
This page lets you generate a testng.xml automatically. You can configure whether to include your test classes individually or by package, the suite and test name and also whether these tests should run in parallel.
This page gives you an overview of the changes that are about to be performed. You can also decide to exclude certain files from the refactoring.
When you are done, press the "Finish" button. Like all Eclipse refactorings, you can undo all these changes in one click: