You can add buildings/structures to maps from the props library so technically if you created a model you should be able to add it. The stone arch on Theta is a prop you can add to maps, It's just a textured 3d model you can rotate and place anywhere. You could line lots of them up in a row and even make a tunnel out of them. You can also adjust how high off the ground it is so if the arch has collision it might even be possible to build a base under them and use it as a roof that blocks bombs lol
To make structures that units can travel and build on i think would require pathing and all that jazz to make it work, not sure if that is possible. Would be cool though, we could add land bridges to rivers that boats can pass under and units can cross.
Edit: Just tested it, you can use many arches as a tunnel and build under them, but they are destructible and bombs destroy them. It does block the first bomb though