Religion.
With their advanced technology (or even just with technology we real humans have currently) your aliens could rather easily convince the entirety of the human population to believe they were gods/angels/supernatural agents.
You canThey could then convince humans to do anything you likethey liked, including but not limited to:
- Not killing each other.
- Develop science and rationality.
- Learn to love, respect and tolerate each other no matter how different.
After they have learned to do former (aided by yet more advanced technology), the aliens can then explain (or hopefully have humanity understand this on their own) that they are not gods, but people from different worlds.
There is no doubt that the human population will be infinitely greatful (and tolerant) towards the aliens since they did afterall enlighten them. Earth can then be seamlessly annexed. No need for any massive cultural/societal exchange, the people of earth have already been intergrating Kroll values since decades/centuries.