Is it possible to combine charcoal and graphite in one image?
Yes, you can mix media. But you need to preserve some of the tooth of your paper's surface to successfully transition from working with graphite to charcoal.
The tooth of your paper determines how well things will catch and hold to the surface of the paper.
As you pass your pencil, or charcoal, across the paper's surface little bits of the drawing tool are abraded and deposited onto the drawing surface. Its the abraded pencil material sitting in the grooves of the paper that form the image. Unlike paint that gets absorbed and wicked into the fibres of the paper the abraded dust particles of your pencil are caught and held by the fibres of the paper. Not absorbed but held in place. This is why very smooth paper doesn't take to pencil very well. A smooth piece of paper won't abrade material off the pencil. No deposited material. No image.
Now the problem with pencils is they are made from a material that can fill and smooth the tooth of the paper quite well. The &quot;graphite&quot; in pencils is made from mixture of graphite and clay. Heavily working your pencil over the paper's surface will cause two things to happen: the tooth of the paper will be worn down and there will be a large deposit of graphite and clay filling the remaining voids in the paper. I have had it happen to me where I have overworked a pencil drawing and the surface of the paper has what almost looks like a mirror finish. The burninshing results in a perfectly smooth surface that nothing can grab onto, not charcoal or paint will stick to these burnished areas.
So in your case if you have been gentle with the application of graphite to the surface of the paper and retained some of the tooth then you should have no worries about working charcoal onto the surface as well.