With shaders and ordinary alpha blending I don't think this is possible. If you wanted to replace all the alpha values in the scene by a global constant, you could use
D3DBLEND_INVBLENDFACTOR. But it sounds to me like you want to multiply each pixel's alpha value by a global constant. This is best done in the pixel shader.
That being said, fading out the alpha of every scene element may not be what you actually want to do. It will produce a strange effect as opaque objects will become transparent during the fade. For instance, an object hidden behind a wall will be briefly visible during the fade, as the wall and the object are both partly transparent.
If you want to fade to a constant color (black, white, etc.), a better approach may be to just render a full-screen pass of that color over the entire scene, rather than altering the alpha values of anything in the scene. If you want to fade to a different scene (like a dissolve) then the second scene should be drawn into an offscreen render target, then drawn on a full-screen pass over the first scene.