Also seems you can't override existing methods or properties still (actually achievable with swizzling of methods & property getter/setters on init but always seems way to heavy to use in reality). I would love to have a more mix-in friendly environment at some point in time.
“You can use extensions to add properties (including class and static properties). However, these properties must be computed; extensions can’t add stored properties to classes, structures, or enumerations.”
“You cannot use extensions to override existing methods or properties on Objective-C types.”
Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C.” iBooks.