hi, some time ago I have been fighting against the same problem: simply put it's not possible with the current architecture of JoomSEF: you get language codes always or never, no grey zone afaik.
I'm almost ok with this limitation of JoomSEF, because I use it for an intranet deployment of joomla, and I didn't really care nor changing the urls neither how they would look like after. I understand however that in a public deployment, changing existing urls can be a disaster (broken links from outside, loss of google rank, the list goes on)
you can try circumventing the problem: for example, introduce custom JoomSEF redirects for italian language, that map your current urls (domain/page) to the new ones (domain/it/page). you need one for each existing page. enable JoomSEF language codes and it should work.
please note that doing so, you commit yourself to using just the \"new\" (domain/it/page) urls in the future. in fact when you will add a content, news or whatever, I'm sure you don't want to remember adding a new custom JoomSEF redirect each time, do you? for this reason, you also want to update all your internal links to the new version.
it's messy, and will break under conditions that don't come to my mind right now, but I'm afraid you can't do much better.
