When going from the stationary saucer hull to the gravity wheel an elevator is needed. This is an unusual elevator in that it must start from being stationary and then begin moving in a circle around a track while following the rotating gravity wheel until it catches up. After catching up, and maintaining a speed of about 100 miles per hour, near 1g gravity takes hold inside the elevator. People can then move down an exit shaft in the elevator, through some airlocks, and into the upper ceiling area of a pod in the gravity wheel. From there, a conventional elevator like on earth can be ridden down to the floor of the pod.
The video above shows the idea of the elevator in the Gen1 Enterprise. The moving part of the elevator system is magnetically suspended to avoid the noise and wear that friction would cause if this moving mass instead rode directly on a circular rail. The technology to suspend and move the gravity wheel can be leveraged and applied to the elevator.
The elevator and magnetic track are inside the donut-shaped cavity that houses the gravity wheel in a vacuum. This is necessary because the elevator must catch up to the moving gravity wheel and then attach to it for transferring people and cargo. This could not be done if the elevator was outside of the donut-shaped cavity that houses the gravity wheel. This also explains why, as mentioned above, that an airlock is needed for transferring people and cargo between the elevator and a gravity wheel pod. An airlock it needed to avoid exposure to the vacuum.