Scripts

There are many ways to do this. Here is one of them:

SetObjectEnabled("garrison_name", nil);
Trigger(4, "garrison_name", "capture_gar");

function capture_gar(hero)
	local a = {};
	a[1], a[3], a[5], a[7], a[9], a[11], a[13] = GetObjectCreaturesTypes("garrison_name");
	for i = 1, 7 do
		if a[i*2-1] ~= 0 then
			a[i*2] = 2 * GetObjectCreatures("garrison_name", a[i*2-1]);
		end;
	end;
	SetObjectEnabled("garrison_name", 1);
	Trigger(4, "garrison_name", nil);
	MakeHeroInteractWithObject(hero, "garrison_name");
	local i = GetLastSavedCombatIndex();
	while i == GetLastSavedCombatIndex() do
		sleep(1);
	end;
	if not IsHeroAlive(hero) then 
		SetObjectEnabled("garrison_name", nil);
		Trigger(4, "garrison_name", "capture_gar");
		return
	end;
	for i = 1, 7 do
		if a[i*2] then
			AddObjectCreatures("garrison_name", a[i*2-1], a[i*2]);
		end;
	end;
	DenyGarrisonCreaturesTakeAway("garrison_name", 1);
end;

Here, you only need to replace "garrison_name" with the script name of your garrison. In the script, it should be written in quotes, and in the editor (in the Name field in the object's properties) - without quotes.