Koelsch is a light, simple beer, a little smoother and less hoppy than pilsner. There are several good recipes to look at, like Ed Wort's
Bee Cave Koelsch, Diatonic's
Black Widow Koelsch, or Tiber's
Perfect Koelsch. You can add Munich or crystal, but it should be with restraint, to keep the color and flavor light (keeping in mind that it's your beer, and you can do what you like; home brewing styles are flexible).
Another route to go, if you would prefer, would be to convert over to Pilsner's more widespread (compared to Koelsch) German rival, Munich Helles, such as Saccharomyces
Muenchner Helles, Joe Dragon's
Augustiner Hell, or Biermuencher's
Helles Belles. There's more scope to incorporate crystal and/or Munich.
Just one more thing - you need to mash the Munich. You can just put however much Munich and crystal that you plan to use in a single steeping bag, keep the temperature around 152 degF for about half an hour, and stir it once or twice during the process to make sure that it all gets wet. Get the temperature up to about 170 degF, lift out the grain bag, let it drain thoroughly, then proceed as you would have.