I just started a second playthrough of this game and I wanted to know if it was possible to acquire allied provinces peacefully. I have a few allies with non aggression pacts. Will there eventually be an option to annex them?
No, there won't. But actually allied kingdoms are sometimes even better than having them as your own - at least if you like diplomacy. Once you take those provinces by force, diplomatic actions with them won't be available to you. And you can cross any allied territory without any restrictions - and later in the game, you will have more provinces than what your heroes can manage, so I would keep them as allies. But you can conquer them of course
While a "diploannexion" would be nice for roleplaying reasons, I agree that in many cases, the diplomatic options (like trade contracts, troop training and recruitment) are actually better that whatever benefit would the ownership of the province give (since there are no taxes in this games and provinces only benefit the liege hero's army).