You will most certainly have the freedom to add your own units and buildings (see: Raap's Hostile Waters). Every thing you may need to learn to do that can be found in the W3D Tutorials section.
Of course, tutorials aren't worth much without the proper tools! GET STARTED - INSTALL THE TOOLS
Game asset creation, character modeling, map making and more! All can be done with time, patience, and the desire to learn and to create!