While there are many things that can be done to alter or even improve the appearance of the face, including filling out "hollow cheeks," all of them would begin with a consultation to a surgeon
trained in facial plastic surgery. Depending on your specific circumstances, age, gender, and even race/skin type, the options would be very different. Additionally, if you have had surgery to your face previously or have a history of either cancer or irradiation, the options would be very different indeed.
There are however, some basic ideas that can be given. Sometimes, relaxing of the normal wrinkle lines and aging can help to soften the face and make sharply hollowed cheeks look less pronounced. A simple way to take care of that is through the use of Botox injections that can done quickly and on an outpatient basis. There are other products that can also be injected to add
bulk to soft tissue (which would also be a temporary fix, like Botox, requiring repeat injections in the future), but generally the final step would include some sort of surgical procedure that would move part of your body in to fill up the hollowing. There are multiple soft tissue flaps that can readily be moved into the area, but all would be extremely delicate procedures. Make sure to find a facial plastic surgeon
with good references before going to one, and a second opinion is often a good idea.