A magician-esper hybrid is a person who can use magic and esper abilities. However, since magic and esper abilities have completely different and incompatible formats, and the fact that espers' brains have been modified due to the Power Curriculum Program, the hybrid will suffer severe damage from performing magic.
The only "successful" hybrid is Tsuchimikado Motoharu, as a result of him joining the Power Curriculum Program in order to complete his ruse of being an ordinary student, him being an operative sent to spy on the city. However, this is only due to his esper ability being able to repair any damage inflicted to him, which could fail at any time. Because of his hybridization, Motoharu, a talented magician in his own right, can only perform magic a limited number of times before being grievously injured.
There was an experiment to create magician-esper hybrids by Academy City which involved sending espers to learn magic, which Sherry Cromwell was a part of. This experiment was a failure not only due to the injuries the subjects sustained from using magic, but the Knights of England killed off the subjects, including Ellis, Sherry's friend, due to the belief that the experiment was against the truce between the magic and science sides.
There is currently no way for a magician or esper to become a hybrid without the dangers present with the incompatibility of the formats between magic and science. Kuroyoru Umidori, the cyborg esper had once abused the negative side effect of an esper using magic to create an avalanche. Being a cyborg, what counted as her body was vague, by imagining something to be part of her, for example such as a cliff; she can force the negative side effect on a target by using any random magic spell.
Since Ranma’s ki isn’t set into a permanent form he should be able to use magic without the negative side effects so eventually he most likely will be considered this. However the problem is that magic in Toaru Majutsu no Index is clerical and for the most part uses Christian ideology so forcing his mind into the right mindset might create a similar backlash effect. Onmyoudou would be the best form of magic for Ranma with the other forms simply being a non entity. As not only is it a traditional Japanese esoteric cosmology but he most likely had to have learned some of its principles such as balance with the elements, controlling the flow of energy
Ranma learning magic would be multi stepped process. First he would utilize Kuroyoru Umidori’s technique. Though she lacks knowledge to cast a proper spell she activates magic to exploit the negative side effect of an esper using magic to create an avalanche, transferring the rejection from her to her improvised cyborg arm, and from that arm to the cliff which it was lowered. Ranma would do the same thing to mitigate the damage on his own body when casting magic.
Once he gets better he’ll move onto how Accelerator uses magic use delicate control to reduce the side effects of using magic so that he doesn’t suffer serious harm. The final by product would be described in Toaru Majutsu no Index as him using magic by utilizing Personal Reality not Idol Theory. Basically Ranma will use Mana instead of chi. Mana is created by processing a person's ki similar to how crude oil is refined into other substances. This will provide him far greater power and versatility than before.
The greatest example of what a Ranma magician-esper hybrid would look like would be the Ranma from Divine Blood whose magic system does work similarly to how it does in Toaru Majutsu no Index. In that fic Ranma is a feng shih and her chi had even been stated to have changed into mana.