you can add a wheel to a boat that has a tiller. My boat is a 26 ft that has wheel steering that was added by some previous owner who convertered it from tiller to wheel steering.
I guess there are pro's and con's to each. Some people prefer a tiller because there is a different feel to it. My wife never really got the hang of pushing the tiller the opposite way you want to go, so she does better with a wheel.
A wheel does seem to take up some space from the cockpit, so that may be a consideration depending on the size of the cockpit and beam. It also gives us something to grab onto when climbing around and a place to hang a drink holder.