Petitions for including additional territory within an existing watershed improvement district may be filed with supervisors of the soil conservation district or districts in which the watershed improvement district is situated, and in such cases the provisions hereof with respect to petitions to organize a watershed improvement district shall be observed to the extent deemed practicable by such supervisors. No additional territory shall be included in an existing watershed improvement district without the approval of the state committee and certification of its inclusion to the officers indicated in section six of this article.