While others would have you velieve that visiing the DMZ and putting one foot in North Korea counts. We’re going to classify that as a hard no. You can easily arrange a day tour of the DMZ from Seoul but this is meerly allowing you a sneak peak into North Korea. On most day tours you do get to go inside one of the blue huts that straddle the border so technically you can say you have visited. If you really want to see North Korea though you will have to join a group tour via China.
This is easily doable from Seoul. Many flights depart Seoul daily for North East China. You could fly to Dalian, Shenyang, Yanji close to the North Korean border, or start your tour in Beijing. From any of those places simple jump on a fast train to the border city of Dandong to meet up with your tour group. From there it’s a wonderful train ride to Pyongyang and the beginning of an unforgettable experience.