Suppose, someone does a PhD and a Habilitation from a non TU9 university.
Is there any practical barrier for him to become a professor in a TU9 university?
Of course you can. Otherwise their pool for hiring would be awfully small.
But don't take my word for it. To provide some evidence we can look at a department at one of the TU9 universities. For example the Computer Science Department of TU Darmstadt (mainly because I'm somewhat familiar with it)
If we look at the list of full professors and eliminate those that have non-German doctorates, we are left with 18 Professors, 9 of which did not receive their degree from one of the TU9. (Though in practical terms almost none of them did, as the TU9 was only established in 2003.)
Yes, you can become professor at a TU9 university with a PhD from a non-TU9 university.