After you have gone through the initial creation steps. It is easily possible to add additional layers, rename them, and order them in the map.
Adding a new layer
Select the map that you want to add a layer to to go to the layer dashboard. In the upper right there will be a button "+Layers" for adding a new layer.
Clicking on that button will bring up the Dataset list, where you can select the dataset you want to add. Select the datasets and click "Ok"
The layer will be added to the top of your layer's list.