It's easy enough to just harvest the yeast (trub and all) in a gallon ziplock bag and then pitch about half of it, if pitching within 2 weeks of harvest. That way you can clean the fermenter. Now, whether the yeast is appropriate for a Kolsch is another question entirely...but it started as a Kolsch yeast, so it should be good to go.Never thought of using a Ziploc bag. Is it sanitary right out of the box or do you need to rinse?
Pitching onto the entire cake is WAY over pitching. It's better to start off with a small amount of healthy yeast thanhaving allot of tired/half-spent yeasties. Use Mr Malty. If you are going to reuse the carboy, first spray the stopper/airlock area with some sanitizer or Grain alcohol and dump 2/3 or 3/4ths of the slurry out.