Is it possible to grant unblockable to a creature that comes into play tanks to Kaalia of the Vast's triggered ability before blockers?
For example, using Rogue's Passage's second activated ability to give the creature that enters the battlefield unblockable so that my opponent doesn't have the chance to block it.
The doubt comes from the fact that, for Kaalia of the Vast's ability to trigger, she has to be declared as an attacker. Do I still have the chance to grant unblockable after the ability resolves and before the blockers step?
Yes you can do that. For Kaalia's ability it puts the creature onto the battlefield attacking during the declare attackers step. After the ability resolves both players will receive priority to act before it moves to the declare blockers step. So yes you will have priority during that time to activate Rogue's Passage to make the creature brought in with Kaalia unable to be blocked by your opponent.
After Kaalia's triggered ability resolves, players will get priority to activate abilities, including Rogue's Passage's second ability (C.R. 116.3b, 116.3d, 116.1b). Even if the stack becomes empty after Kaalia's triggered ability resolves, the declare attackers step won't end as a result. In general, the step will end only if all players pass in a row while the stack is empty (C.R. 500.2). See also this thread.
For example, using Rogue's Passage's second activated ability to give the creature that enters the battlefield unblockable so that my opponent doesn't have the chance to block it.
The doubt comes from the fact that, for Kaalia of the Vast's ability to trigger, she has to be declared as an attacker. Do I still have the chance to grant unblockable after the ability resolves and before the blockers step?