Fresh water moving through the Great lakes system eventually ends up in the Atlantic ocean via the St. Lawrence river. A relatively small amount of this water could travel via pipeline from lake Michigan to recharge the Ogallala aquifer. The pipeline would only have to be around 500-600 miles (800-965 km) in length. Oil pipelines have been built that are much longer than this.
Needless to say, this option would require some political clout to accomplish.
Every few years, flooding of the Mississippi river results in millions of dollars of damage, loss of life, and other tragedies.
Planners have become very effective at predicting these floods, and will get even better in the future. Why not divert some percentage of the water from tributaries of the Mississippi river into the Ogallala aquifer during years when floods are predicted? Diverting the water would lessen the effects of the flooding and also add more water to the Ogallalla (a win-win situation).