Zone Interfaces Query

Usually what restricts adding interfaces to a zone is if the interfaces have any other reference to them, say from a security policy, policy route, dns, etc.  

In the network interfaces pane you'll probably see a non-zero number for those interfaces under the "References" column.  If you click on that number it should list out the references for you.  Then you need to remove the references before you add the interface to the zone.

If you don't want to just delete more complicated referring objects, like a security policy, you can instead just temporarily replace the reference to the interface you want to add to the zone with a reference to another unused interface.