Thanks a lot for your support. Actually I have simply backed-up my old website (which was plain HTML) and then I have recreated my domain folder.
When I have tried to one-click-install Wordpress into this domain, I received a message like "Some of your domains may not be listed because they do not have PHP installed" (or something similar to this).
Easily I was able to reinstall PHP (used the default option) into this folder, and voilá, I was able to one-click-install Wordpress into the new folder.
Now I have realized that is really simple to create a new domain and have as many blogs into my Dreamhost account as I wanted.
Anyway, thank you very much indeed.