Can you use Paint or some other program that comes in a Window installation to generate/edit the alpha channel of a picture?

Yes, you can use Paint to edit the alpha channel of a picture.

Use GIMP to extract the alpha channel of any image as a grayscale image. Then edit that image on Paint. When you are done editing, use Krita to replace the old alpha channel of the original image with the new alpha channel produced by Paint. You can use Blender then, to visually check the final results by applying the image as a texture to a quad.

that`s too many stages. I`ll have to come up with a solution of my own.

Microsoft Paint 3D (which actually replaced Paint entirely in some Windows builds) can do some basic stuff with an alpha channel. But I don't think you can directly edit the alpha channel as a separate image independent of the colour channels.

@nikito

Can't you do all that within gimp directly using a layer mask?

SyncViews said:
Can't you do all that within gimp directly using a layer mask?

Choosing the easiest path is always the wrong choice.