Any area not within a highway district, but within a territory previously detached from a county and annexed to another county, and adjacent to a highway district organized before annexation and situate wholly within the county to which the territory has been annexed, and any area not within a highway district, which is adjacent to an existing highway district and situate wholly within the county within which the highway is located, may be added to, and included in, the highway district upon the approval of its highway commissioners and the order of the commissioners of the county in which the highway district is situated.