Can hair be transplanted on to scalp birth mark which is devoid of hair.

You can transplant hair to a birth mark in general.  But this all depend on a medical exam and a clear diagnosis.

First, see a dermatologist or plastic surgeon to make sure the 'bald spot' or 'birth mark' is not a skin lesion that needs to be removed. Nevus sebasceous is a skin lesion, present in infancy, that is hairless, and can become skin cancer (BCC) later in life. Assuming there are no issues, then hair can be transplanted into this region from an occipital scalp donor site. Neograft FUE is a good option for hair restoration.

It would be helpful to see a picture of the birthmark. There are several different types of 'birthmarks'. Vascular birthmarks which are usually red or purple would probably not be suitable. Likewise it might not be sensible to do a hair transplant into a pigmented (brown) birthmark.