There are actually a couple of ways of handling this. You've identified the harder way. From our panels here at DH, we can just add hosting for domains that are registered elsewhere. This creates the virtual host entry in the Apache of our server (and sets a bunch of other things up). From our panel, we can get the IP address for our domain. With this info, you will be able to do exactly what you said above.
There is another, easier, way. One of the other things that happens when you add hosting via the DH panel is that the DH dns servers add the DNS entries for your domain. This means that they can serve as the DNS nameservers for your domain - providing info about email servers, server IP addresses, etc. This is what everyone else is alluding to. To do that, all you need to do is to set the nameservers for your domain at your registrar to ns1.dreamhost.com, ns2.dreamhost.com, and ns3.dreamhost.com. When you add your .it domain to your hosting account here at DH, everything will "just work".
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They're Dreamhost coupons!