It's not ideal, but as long as most of the diet is actually dog food, it won't hurt the dog. It would be more harmful to feed dog food to a cat on a long term basis.

My dogs love canned cat food. If I have one who's been ill or won't eat for some reason, a can of stinky cat food on top of their kibble gets their appetite going almost every time!

Canned cat food works great for hiding pills in too!