Signal extension Using the Command Line!

I would like to find a way to extend a signal to a desired length. I also want to do this by using the Command line and NOT the Graphical Interface. For example my initial signal is x = [1 2 3 4 5] (which has a length = 5), I want to extend it so the final signal has length = 512. I tried "wextend" command but it does not give the ability to define the final (desired) length but only the length of the extension. So anyone has an idea of how to do this? i would be grateful if anyone could help me!