I really wish the SourceForge forum let one edit their posts to correct mistakes.

This is the Sweet Home 3D forum. You can edit your posts for 3 days (I think). (see at the bottom of this post: it shows that it is edited.)

I wonder can this plugin tool be used to subtract from or slice into a wall itself?

Not the plugin, it requires furniture objects. But you can export an object and import it as "Door or window" to cut into the wall. This will however cut completely through the wall, not an indention. You can export a mannequin and import it as "Door or window" and create a cutout in a wall with the outline of the mannequin.
But you can always export the wall and import it. Then it is a furniture object that works with the plugin.