Pietro,
As we all know, the TDA does not provide official rules or guidelines on bounty tournaments.
IMO: Tournament structures can include bounties, reentries, rebuys, add-ons, etc., in as many varieties as one can imagine. Obviously, most important is that tournament rules and structure are made very clear before players enter the tournament. Of course, finding a structure that pleases all players is likely impossible.
Most often, (as Dave said above,) a bounty is included with each rebuy or reentry. However, I have seen rebuy tournaments where only at the end of the rebuy period an additional add-on/bounty is offered to those who are still in the tournament. To win a bounty, one must have purchased the add-on/bounty. If a bounty player is busted by a non-bounty player, the bounty chip is carried into subsequent pots until a bounty player wins. This format is similar (but different) to a last-longer prop bet.