As far as I know, it's not possible to add additional buttons to the interface, but those hero buttons. So there are only a few methods to create some "buttons" for you in the upper right corner of the screen.
1. create a multiboard and try to create a button there. like choosing those small icons, or a textfield that looks like a button or whatever.. you can trigger the "click" part then.. but it's only possible with keys, like the arrow keys, etc.
2. change the interface itself. I don't know if this is possible, but you could add buttons to the interface directly and import them into the game. but the negative part is, that you can't change the button, it will be the same for the whole game. and it's not removeable ingame.
3. You can add buttons via, images that you place on the ground and then use some specific camera angle point to view on that image from top. make some model-less unit and place it above/under that image, so you have something to click, the rest works with triggers. the negative part here is, you can't have those buttons while playing the game.. so you have to move to that button again and again, before you can continue playing.
So it's not that simple to add a Button InGame like you want.. it's not possible to do that, like you want it do be, like the Hero Icons on the left side of the screen. hope this helped a bit
