like i could create archers or trebuchets from barracks?

Yes I don't know the specifics but you use this program to do it:
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Program is called Advanced Genie Editor.

In general AOK Heaven Games is a better resource than asking questions here.

This would be a mod though so you'd have to get people to install the mod before they play the scenario and probably get the mod verified with Donnie/someone else here.

interesting, so there are not ways to do it with trigger i understand?

That would require modding. If you want to avoid that you would need to set up a store system where they bring a king to an area and then the unit gets created.

No, using triggers is impossible OBVIOUSLY unless as Rambit told you... it's just "Imaginary" creation from there, not by clicking in the building and then in the icon of a trebuchet.