Petitions for including additional territory within an existing watershed improvement district may be filed with directors of the soil and water conservation district or districts in which the watershed improvement district is situated, and in such cases the provisions hereof for petitions to organize the watershed improvement district shall be observed to the extent deemed practicable by the district directors. In referenda upon petitions for such inclusion, all owners of land situated in the proposed additional territory shall be eligible to vote. No additional territory shall be included in an existing watershed improvement district unless owners of land representing two-thirds of the acreage proposed to be included vote in favor thereof.