Can anyone guide me on how to create a new category that allows me to place previously created categories below the new one?
 
For instance I have an existing main product category P, category Q, Category S. Now I wish to place all these 3 under a New main category A. How do I do that?

Easiest way, is to create your category, and then go into P,Q, & S, and from the drop down menu, select the "Parent Category".
 
HTH

Thanks, worked great and so simple.