Set transparent color in sprites ?

Hello, how you can set the transparency color for Spite ?

 

Unity wants alpha channel for transparency..

Not sure if there are some plugins for doing that inside unity..
(would be nice to have actually, so could use all the ready sprite sheets from internets)

 

@mgear I want something similar to this:


How can I convey the wishes to developers?

 

Can send 2D ideas here,
https://bitbucket.org/Unity-Technologies/2ddemos/wiki/Home
http://forum.unity3d.com/threads/2d-alpha-release-4.359622/

and vote here,
https://feedback.unity3d.com/suggestions/color-to-alpha-transparency-in-sprite-editor

 

in the meanwhile, this program might help,
Creates transparency in images which come with a single key color:
https://www.codeandweb.com/texturepacker

*actually this editor script works,
http://answers.unity3d.com/answers/252528/view.html

here is updated version,
- automatically enable [x] read/write for input textures
- make it faster by displaying progressbar only every now and then
https://gist.github.com/unitycoder/87dd17a123b008a251b9

 

Any update on this?