Thank you for your question!
Yes, you can add spaghetti sauce to the Mac 'n Cheese beef bake. An important thing to keep in mind is that when you add spaghetti sauce, you are adding liquid, so if you do not want the mixture to turn out too wet, you need to remove some of the other liquid in the recipe. For example, the below recipe includes milk. You can follow the directions as normal, but substitute half of the milk for that amount of spaghetti sauce. The sauce will be slightly thicker, so you can add a bit more milk if a thinner consistency is desired.
Answered 03/12/2013 by mgoetz