
Pep, I got a chuckle out of the top-secret blurring in that pic. Maybe I'm too easily amused
Omen: You CAN make a subtracted space over added space, but after doing so you need to right click on the new subtraction brush and set Order>Order To Last. This will tell the editor to build that brush before anything else within or in contact with it. But as Pepito said, it's better subtract first and only use this re-ordering trick to get out of trouble.
Also you could use the Vertex Editing tool (above and right of the scaling tool) to select and drag the corners of the brush to their new locations. This will keep the brush on the grid and allow seperate lateral and vertical control of where the brush moves to. It's also a great tool for changing simple brush shapes into sloped or angled shapes to get away from square boxey geometry. Only thing to be careful of is dragging a corner out of plane with the others thus twisting the face and probably creating a nasty BSP hole. Hold Ctrl+Alt+Left Mouse button and drag a box over the points ya want to move, then Ctrl+Left Mouse to drag them (I think).
******EDIT******
I was beaten to the punch, but I'll leave this in just in case it helps somebody else
For your sky, it sounds like you are placing the star texture on the walls of the box you are building in. Better technique is to set the surfaces where you want your sky to be seen as Fake Backdrop in the surface properties Flags section. Then build a seperate "Skybox" outside of your play area with your whole sky scene- Star texture, moons, nebulas..... now place an Info>ZoneInfo>SkyzoneInfo actor in the skybox and it will show the sky scene on all surfaces flagged to Fake Backdrop in your level. I would set the star texture to Unlit for a realistic look with no shadows in the corners. After re-building, go in the play area and check out the sky by hitting K (hotkey for Show Backdrop).