To have a drug travel to a specific place in the body can you just coat the drug in extracellular signals recognized by target cells?
Yes you can, in principle, coat the drug particle with ligands such that it will be targeted to specific cell types. This is being considered and actively worked upon for development of targeted drug delivery. However, you should also bear in mind that doing these kinds of coatings would be costly and it would greatly increase the cost of the drug. 

At least, in the case of cystic fibrosis or any other disease of the lungs, an easy method of restricting the drug to lungs would be by taking the drug in the form of aerosol, using inhalers. 




  Mechanism of inhalers.   Picture source: http://www.globalasthmareport.org/management/inhalers.php