Hi,
try to add a menu with user ACL and add k2 item creating link in this menu.

Maybe I've understood your advice.

1) I done login with K2
2) I copied the URL of the K2 link "Add new item"
3) In the backend I created a menu item of type "External URL" and I pasted the URL
4) In frontend I done logout from K2 and Login from CB

Now the URL open a page with K2 Editor and so I'm able to write an article with categories, tags, extra fields, etc
But obviously when I save the article, there is nothing to close the window and so the editor is always open with the article.
I have to open URL in a new window of the browser and then to close this browser window.

Your advice is surely a big step forward (thanks!) but it is not the better solution.

A possible alternative is to publish both Login modules, CB Login and K2 Login with this last module showed only with access to "Registered".

In this manner I've only the CB login and after the login I can see the "K2 login module", but with user already logged and so able to see the 4 K2 links.

I have two "logout" button, in CB module and in K2 module, and the link to modify K2 profile (and not CB), but these are minor problems.

However it is strange that with K2-CB Integration it is out of the door the main feature of K2: to write articles.
Furthermore I can show CB profile, but if I ask to modify values it is showed the list of the k2 fields.

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Last minute news.
In the k2 forum I have the confirm about this, it is mandatory to use K2 Login Module to add articles, also if after a CB login.

Thanks to everybody

K2 Forum confirm that I have to copy the link from K2 Login Module and past it in the User menu. After this it is possible to disable the K2 Login Module.

At the moment there are no other solutions.

Thanks to all