It depends on the classes. You could grab JDK6 classes from the OpenJDK6 and put them on your bootclasspath but:
1. It can't be any of the really core classes that are intimately connected to the VM (Object, Thread, Class, ClassLoader, java.lang.ref.*, java.lang.reflect.* ...)
2. They can't require VM support that only exists in JDK6. For example a JMX API that only exists in JDK6 will need support from the JDK6 VM.
3. There can't be any dependencies between classes such that JDK5 class X plus JDK6 class Y leads to breakage.
Of course this is not officially supported.