It's complicated and impossible in Krasic's case. First of all, it's not about who buys the player but who registers him. We can buy Krasic right now even if we bought and registered Dzeko 1 day earlier, but we can't register Krasic.
So, some other Italian team will have to register him first and then we get him from them.
But the problem is, 1 player, in one season, can be registered for only 2 clubs. Krasic is already registered for this season in CSKA. If Siena register him that will mean that until the next season he can't be registered with another club.
Also, it's not that easy to find a team willing to lose the non-EU spot just to later give the player to us. Although, it's nothing a couple of loans of youth players can't fix. It's not like Siena buy many non-EU players.