Fabricio
I understand what you mean.
In this case, I suggest to have a look at the Flagrow byōbu extension, which allows for private discussions. With this extension anyone can set up a discussion with a freely selected number of participants. Participants can be either individuals or user groups. In your case the said user would have to select himself plus the moderators' group. The discussion can also be initiated by a mod with the same settings. Tags are not needed in this case.
Unfortunately, users would need to know these steps to start this kind of conversation with moderators. Until now there is no solution for a simple "chat with moderators" button.