Write a Editor to select file from disk and import to unity

Fact is Unity won't let you do that directly. There is this:

http://docs.unity3d.com/Documentation/ScriptReference/EditorUtility.OpenFilePanel.html

which only work in Editor mode meaning you won't be able to put that in a build.

You may think it is exactly what you need and wonder why they would not allowed that in the build, security. I could sell you an application that on the front allows to save and open file from your hard drive, but in the meanwhile, on the back, I am scanning your computer, sending all your personal pictures (even the kinky ones :)) and sending them to my blog.

Now there are solutions though, first if your game is a web player, you can use Javascript, the real one, the web one and make a small method

http://stackoverflow.com/questions/6463439/how-to-open-a-file-browse-dialog-using-javascript

that opens up a file explorer, the method is then called using Application.ExternalCall.

Otherwise there is this: http://wiki.unity3d.com/index.php?title=ImprovedFileBrowser