By "layover" you likely mean "connection". Whether and what this will cost, if anything, is a matter of the fare rules for your specific ticket. Best thing is to determine the routing you do want and then calculate what it will cost to make the change. It might not be that much.
While KL does not serve PIT, DL most assuredly does and there is service on KL/DL connecting at JFK and DTW. There may also be other more circuitous routings and those may be even cheaper (or not).
Bottom line is that hypotheticals do not mean much when faring a ticket or a change.