You can change the flights....you can go through a change, then use the search to book a multi-city. Or just cancel the itinerary and search a new flight, then use the credit to pay (plus any additional balance). Either way, the fare should be the same, but note you are now doing a stopover, which is different than a layover and will break up the fare - depending on the fares it theoretically could be the same/cheaper, but is usually more expensive, as you are either going to be on two separate fares, or a single fare with an additional fee for a stopover.

On the other hand, it's never a bad thing to price out booking these as 2 tickets, as it could also be cheaper that way. Is there a reason you don't want to cancel and book 2 tickets?