Any county road which is not a part of the state highway system, but which road is a continuation of a state highway, which state highway is a cross-state highway except for the part of said road which is not a state highway, and which part is not more than fifteen miles in length, and where the inclusion of the said fifteen miles of road as a part of the state highway system would secure the improving of a continuance of the road in an adjoining state through reciprocity legislation and thus make a continuous highway across not less than two states; and providing said county road where said road is located is brought to standardization by the county and approved by the secretary of transportation and providing said adjoining state completes its link of road which is lacking in its state highway system to join with the said Kansas road, then the fifteen miles or less of road not now in the state highway system of Kansas shall be made a part thereof and shall be placed under the supervision and control of such secretary: Provided, That this act shall be supplemental to Kan. Stat. Ann. § 68-406.