Question about Lenovo ThinkPad P51

Basically not.
If you look at the notebook from all sides, you always see fan outlets or connections somewhere. Nowhere is there an expansion slot or the possibility of squeezing in a drive somewhere. It's built and designed from the ground up so that it will never have an internal drive. Besides, it's completely alien to me that you can retrofit a drive into a notebook. They are usually loud, slow and not space-saving, which is why people try to get rid of them.