Getting troops into fortress during the attack

Yes you can. You may have to time their arrival so that the enemy army is not attacking while your reinforcements are within melee range, but not joined to the garrison in the fort. Usually that is not the case and you can delay their arrival so that they will only enter melee range after the attacking army has attacked for that round (if needed). Also, make sure you send them an order to move to the fort, not attack the enemy there.

What will happen is they will slip into the fort between attacking rounds and join the garrison force.

Thank you Hannibal for your answer!

On the other hand, what will happen while your army is fighting in a battle as a attacker and want send reinforcement in your stack for joining to your stack but not attacking the defender as a different stack ? ( I mean, join and attack in the next round)

For example, I was attacking with my stack and the enemy's allie came to help him as a attacker stack, so I needed more troops there. I wonder how can I send backup there without start attacking them as a separate stack??

This comment comes from a player not a staff member. What I am about to say is from my experience not my membership on staff.

there seems to have been a fix recently that causes merging troops to attack before falling into ranks. Sometimes there is enough HP to survive and the remaining fall in but most times the single or small group can't handle the initial battle and just dies.

What Hannibal said worked in may 2023 when he commented

Again me commenting on what I think.

This by itself would not be such an issue if the 5K area surrounding the fort was not removed. Troop attempting to merge, but being denied due to updates are as good as dead without this protection and ability to merge SUCUSSFULLY in the first attempt.

Bringing small groups or Lemmings into combat range aka splash damage range of 5km will cause units to initiate thier own attack rather than merging with the garrison, we used to go to diplomacy settings and change setting from war to trade embargo to get units into garrison, I read in some update that one party changing diplomacy setting is not valid manipulation after some recent update, design team is usually doing busy work with graphics, I am astonished to see them adding changes to game mechanics after cloning CoW and putting on CoW uniform for this alleged WW1 strategy game...

To compensate for the Hit points you can execute double attacks against your opponent if the supporting units have enough hit points to survive the initial damage, I have not tested this new udate so I wonder if merging units will get the fort deflection of damage?