Does the existing system have flex tubes routed into the towers? If so, once you bridge the gap between the towers and kegbox, reinsert these back into the tower.
Home Depot / Lowes would have everthing you need. Dril hole through your bar top the same size that is in the top of you keg box. Wharever you use to bridge between the two, be certain to insulate the outside.
Scott Zuhse, Instructor
Micro Matic Dispense Institute