Buddha's knowledge or Wisdom which cannot be achieved by his disciples comprises six kinds. They are called the Saya Asadharana Nana. Did the Buddha attain them at the same moment he attained Nibbana? If so, are they part of Nibbana and are they considered as Asankara as well?
Yes, the Buddha attained each of these knowledges at the moment of enlightenment.
No, the knowledges are mundane, and thus sankhata (formed). They are a by-product of the Buddha's specific enlightenment, not qualities of nibbana itself, which has only three qualities, viz. voidness, signlessness, and desirelessness.