Can veneers be placed on existing front teeth crowns?

Unfortunately not. In order to prepare a tooth for a crown, you would have to shave all around and on top of the tooth. Basically your tooth would become a smaller version of itself and the crown would cover it and replace the missing tooth structure. When the existing crown comes off the tooth underneath is ground down all around and needs to be covered again with a new crown. The veneer would only cover a part of the tooth and the rest of the previously cut tooth structure would be exposed.  So you would have to replace a crown with another crown only. 

No, you cannot do veneers once you already had crowns, what you need is new crowns. zirconium would be great choice, metal free great resistance. you can change shape, size and even color.