That is one way. Another is to boil some hops in a DME mix with 1.040 gravity for about an hour, cool and add to your carboy. How much volume you will need for that is beyond me.
A tea of water and hops would work, it just wouldn't be as bitter (someone may need to second this opinion) and would dilute your kolsch.
Kegged (as of 5/14): Rye IPA, Honey Sage Ale, Belgian Golden Strong, California Common, Bavarian Style Lager, Weizen, Coffee Stout
Long range forecast: Douze clone, Gruit