send one of your daughters to a foreign general and you can propose a marriage, i believe. I've haven't tried it because its too easy to just slaughter every living thing on the continent. make war not love you dirty hippie.
^that or...well what do you mean by arranging marriage because the entire game IS about arranged marriage. Even if you dont marry your princess to an enemy's general or faction heir/leader, you are still going to pair her up with some dude from your own faction. The only exception to this is of course if you never pair her up and she ended up being in a convent (resulting in her being all weepy for some reason. I guess princesses dont like becoming nuns.)
Ahem anywhosome, yes if you want to pair up your princess with an enemy general (or your own general), do the above. If you want to marry her to the enemy faction leader or heir, you can do it with a diplomat or her.
You think that Fortune changed in her relation to you; you are wrong. These are always her habits, this is always her nature. -Philosophy