Foreign would depend on the definition, if a king has more than one kingdom title, he might sell a barony from one kingdom to a vassal or courtier from another kingdom (it might be unpopular though); another option would be a ruler in a border area with possessions on both sides of the border, but then he would be a vassal in two kingdom and thus won't be a foreigner (he could buy it in his role of vassal of the same kingdom).