Sometimes, an elite foreign aristocracy itself becomes the ruling class. It's happened rather often.
A more 'civilised' kingdom grows, its dominion expands, the court has too many princes to accommodate, some of them venture out to foreign lands with a retinue of loyal followers. Their expertise in politics and administration is appreciated and sought after by a less advanced people. The leading prince ends up becoming king and founding a new kingdom.
If the new kingdom is near enough to the old kingdom, it becomes a vassal-ally of it. Several new kingdoms, together with the old kingdom at the centre, then coalesce to become an empire. A peaceful expansion.
Indeed, if I recall correctly, citizens of the first Russian kingdom, Kievskaya Ruska (Kievan Rus), i.e. Kiev in modern Ukraine, due to fierce, endless local inter-necine rivalry, at one time invited a group of Vikings, whom they called Varangi, to reform their kingdom, with a Viking prince becoming their new king.
While the ancient kingdom of Mittani in upper Mesopotamia was said to have been ruled by an aristocracy of Indian origin.
Conquerors adopting the culture of the conquered, themselves getting assimilated in the process, of course, is a frequent occurrence, the Hunno-Turko-Mongols being an obvious example.
Edited by Shield-of-Dardania - 02-Jun-2010 at 23:33