Buy PSN games and DLC without giving Sony the credit card info

I have done that myself.

Since i'm not from USA, i created a different account i buy credit for it to buy some stuff. The only restriction is that in order to play or use the stuff you buy with that account, is that both are as users in the same PS3 system.

This is necessary since it has to check for the user who bought it, but you can freely use the games/stuff for your main account

Unfortunately, this is not possible, at least according to the UK site. (I assume the same is true in Italy; they don't seem to have the same screenshots on the Italian site, though.)

When you create a Sony Entertainment Network account, you set your country of residence - in your case, Italy. This can't be changed later, as indicated in these screenshots. Because the country in the billing address for any forms of PSN payment must match your indicated country of residence, you can't add a credit card from any other country or change the country of an existing credit card.