Thats easy! Convert your object to an editable mesh. Then you will select vertices and create as many vertices as you want, wherever you want. To connect thos, you should select "Faces" under Editable Mesh modifier. Click on "Create" below. Then you will need to connect 3 vertices in order to create a face. If you connect the vertices Clockwise, they will be facing behind, and you won't be able to see them, but if you connect them counter clockwise you will see the face, because its facing is towards you. This Backface culling is to save memory.
But, if you need both sides to appear, you should edit the material, as Warcraft III, and check 'Two-Sided', this will ignore the backface cullling, and you will be able to see both the sides in-game.
But, stil, you want to see the face from behind only for diting purposes, you can right click on your view port and go under properties, there will be an option named "Force 2-Sided", check this, and you will be able to work seeing both sides of the faces, but, in-game their backfaces will be hidden. This is sometimes not recommended, because you will be mistaken once or twice at the vertices flow of the faces, and you won't detect it before...
Hope I helped!