You can use env_customize to do it. Here is an example script.
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trigger_auto
target: customize_01
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monster_human_grunt
targetname: soldier_01
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env_customize (first)
targetname: customize_01
target: soldier_01
mi_iPlayerReact (reaction to player): friendly unless provoked
This is for default behaviour.
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trigger_once (for the specific zone you want)
target: customize_02
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env_customize (second)
targetname: customize_02
target: soldier_01
mi_iPlayerReact (reaction to player): enemy
When you get through the trigger_once, soldier_01 (monster_human_grunt) will start attacking you. See SOHL's entity guide (especially trigger conditions for NPCs part) to learn more about variations.