Yes, they most certainly can! In fact, adeniums are one of the most commonly grafted ornamental plant species. Flat grafting and bud grafting are commonly used for propagating adeniums via grafting.
Saplings which are less than a year old are used as the stalk. A six inch long stem which is cut out from another variety of adenium, is used as the scion. The one selected as the stalk, should be cut four inches above the ground. Make a 2 cm deep V-cut on this portion. Cut the bottom of the scion to fit the V-cut portion of the stalk. Now, cover the graft with a thin plastic cover and move it to a shady place. Once the scion has started growing, make sure that the plant gets at least six hours of daily sunlight. It is possible to graft three to four adeniums in a single pot but proper care should be given to ensure their survival.