glTexSubImage2D with blending + scaling

Is there any way to have glTexSubImage2D() apply the replacement texure into the old texture with blending? Like if the new texture has an alpha channel and I want to blend it onto the old texture. How can I do that?

Also, is there any way to set the scale of the replacement texture?

-Noah Desch

though its not exaclty what you want a similer effect could be done through multitexturing