As others have said, this is bad practice, but if you don't have a choice because you need to integrate with a third-party library that uses the default package, then you could create your own class in the default package and access the other class that way. Classes in the default package basically share a single namespace, so you can access the other class even if it resides in a separate JAR file. Just make sure the JAR file is in the classpath.
This trick doesn't work if your class is not in the default package.