Not sure what you're asking exactly. If you mean a new wall with a "NODRAW" texture, then you can't do that because map geometry has to be compiled into the BSP.
You could simulate it by spawning an invisible model and placing it in the correct location. There are lots of examples of spawning models on the forums. You might be able to set the alpha to 0, or if not, then you'd need to create your own "wall" model with an invisible texture.