if I am understanding the question correctly, you're looking for a free stopover? Or you just want to include another destination in your trip? In general you can only do a free "layover" up to 24 hours. Anything longer than that will require separate tickets on most airlines. There are a few airlines that offer free stopovers at their hubs like Korean Airlines at ICN, Singapore AIrlines at SIN, IcelandAir at KEF.