You can add a parent map in two different ways, depending on which is more convenient for the situation.
Be sure that the Offer map inheritance/extract on drag/drop in map editor preference is enabled (it's not enabled by default). Then in the map editor drag an element from the input to the output where the input and output inherit from structures corresponding to a map that you want to inherit from. If there are maps available with the same (or compatible due to structure inheritance) input/output structures, they will be offered for map inheritance.
Expand the child map in the Repository navigator, and then either drag the parent map to the Inherits From tree node that appears below the child map, or use copy/paste to place it there. When you do this you will be prompted for the input and output child map elements to anchor the parent map.