Aside from reconstructive surgery, there is no way to ONLY gain weight on your face unless you tend to be the type of person whose weight gain normally shows up on your face first. Different people have a tendency to gain weight in different areas (where the weight usually shows up first.)
If you want to gain weight in your face, you will have to put on weight in general, and hope that the increased weight will cause your face to fill out. If you are concerned about gaining weight in other areas, you can focus on exercises to maintain those areas as much as possible, however exercise AND diet are both important for weight maintenance.
Collagen injections are mostly used to fill in deep wrinkles or fill out scars in the face, however some people have had success with collagen injections filling in sunken cheeks. Below is a website from the American Society of Plastic Surgeons. This site is very informative and explains different treatment options (with collagen injections, fat injections, etc), risks, and what you can expect your outcome to be. There is no mention of cost, but I would expect that to vary from doctor to doctor and geographical areas.
Please let me know if you need further info or clarification.
During my research regarding your question, I had found so many different opinions to "how to gain weight" only in your face, and there were literally hundreds. I can only tell you from the physiological point of view that increasing dairy products such as you mentioned...cream cheese.... does not have effect on weight gain UNLESS you consume it in excess. We all need a certain amount of dairy, fat, carbs, fats & vegetable & fruit to maintain a healthy diet per the "food pyramid." If you consume too much dairy, you could be at risk of increasing not only weight gain, but cholesterol and dangerous levels of "bad fat" in your blood (LDL.)
I do not believe that 2 teaspoons of cream cheese will impair your health unless you already have a high level of cholesterol...so go for it. It is however, important to share this information with your physician. Unless you already have a high cholesterol level, go ahead and try this method you've learned about. Just be careful that you are aware of your blood levels of cholesterol and fat BEFORE you begin and diet change. Diet has a HUGE impact on health and cardiac status.