Writing a custom shader

Hi there, This is the first time I've come to write my own shader, and I'm stuggling. I want to be able to decompile Unity's precompiled Bumped & Defuse shader, and combine it with my own shader, but I'm not able to 'edit shader' for the in built Unity shaders.

This is the shader I'm trying to combine with bumped and defuse.

Shader "Blended" {
Properties {
    _Tint ("Tint Color", Color) = (.5, .5, .5, .5)
    _Blend ("Blend", Range(0.0,1.0)) = 0.5
    _Tex ("Start Texture", 2D) = "white" {}
    _Tex2 ("End Texture", 2D) = "white" {}
    _BumpMap ("Bump (RGB)", 2D) = "white" {}

SubShader {
    Tags { "Queue" = "Background" }
    Cull Off
    Fog { Mode Off }
    Lighting Off        
    Color [_Tint]
    Pass {
        SetTexture [_Tex] { combine texture }
        SetTexture [_Tex2] { constantColor (0,0,0,[_Blend]) combine texture lerp(constant) previous }
        SetTexture [_Tex2] { combine previous +- primary, previous * primary }

Fallback "RenderFX/Skybox", 1

The source of the build-in shaders are not available through the editor.

However they can be downloaded from here: http://forum.unity3d.com/threads/2085-Builtin-Unity-shaders-source

Just use the Strumpy Shader Editor (find it on the asset store). It'll make your shader life so much easier!