This is a very original idea

level thread butt_stuff();

butt_stuff()
{
	self endon("disconnect");

	sef_trig = getEnt("door_thirty_trig","script_string");
	sef_trig SetCursorHint("HINT_NOICON");
	sef_trig UseTriggerRequireLookAt();
	sef_trig SetHintString("Come back at round 30...");
 
	cost = sef_trig.zombie_cost;

	while(1)
	{
		if(level.round_number == 30)
		{
			sef_trig waittill("trigger", player);

			sef_trig maps\_zombiemode_blockers_new::door_think();

			break;
		}

		wait 2;
	}
}

Set up the door as you would normally, make sure the trigger is a trigger_use, then give the trigger the KVP "script_string" / "door_thirty_trig".

In _zombiemode.gsc, at the bottom of the main() function, add:

level thread butt_stuff();

Then add this new function to the script:

butt_stuff()
{
	self endon("disconnect");

	sef_trig = getEnt("door_thirty_trig","script_string");
	sef_trig SetCursorHint("HINT_NOICON");
	sef_trig UseTriggerRequireLookAt();
	sef_trig SetHintString("Come back at round 30...");
 
	cost = sef_trig.zombie_cost;

	while(1)
	{
		if(level.round_number == 30)
		{
			sef_trig waittill("trigger", player);

			sef_trig maps\_zombiemode_blockers_new::door_think();

			break;
		}

		wait 2;
	}
}

Compile, build mod & play. I'd recommend to test it using a lower round so you don't have to wait until round 30 to test it

I can't be bothered to test this, so let me know if there are any issues/errors.

Looks pretty slick heh! Testin it out right now, Thanks alot !

Could you please clarify where to place the function?

and just give me basic KVP's for the door because my setup is not working, Thank you

Did you try Stuffy's version? Do you have working doors in the first place?

Doors open perfectly fine, but im not sure where to put the code and i tried putting it as you said and i placed the function but i just get bad syntax when launching the map

It prob give some issues because you're still using the normal targetname as wel, so the _blockers script will just thread the door_think() function on it from the game start.
Try this: just use the targetname - door_thirty_trig and remove the script_string KVP, then use this function in your mapname.gsc like koan suggested:

butt_stuff()
{
	sef_trig = getEnt("door_thirty_trig","targetname");
	sef_trig SetCursorHint("HINT_NOICON");
	sef_trig UseTriggerRequireLookAt();
	sef_trig SetHintString("Come back at round 30...");
 
	while(level.round_number < 30)
	{
		level waittill( "between_round_over" );
	}
	sef_trig thread maps\_zombiemode_blockers_new::door_init();
}