I have existing receptacles on the kitchen wall and the boxes are flushed with the wall. Well actually the boxes themselves are flushed with the stud faces plus a mud ring that adds another 1/2" or so to make them flushed with the wall.

However, I need to bump them out another 1/2" so they will be flushed with the back face of the kitchen cabinets.

I know there are all sorts of adjustable plastic box extenders. Are there any extenders that will do the job for one or two gang metal boxes? I can remove the mud rings but the standard metal extension rings are at least 1-1/2" deep.

 

You could use deeper mud rings or just leave the boxes as they are and add electrical outlet box extenders. They come in one, two or three gang.

 

If you can remove the mud rings, just install deeper mud rings. You should be able to get 1" or so from the home store but I have installed up to 1 1/2" in the past.