Dentists give a wide variety of cosmetic appearance procedures using botox, etc. when will they start plastic surgery as part of their practice?

Probably never: To become a Plastic Surgeon requires completing Medical School and then a residency in Plastic Surgery. Completing Dental School makes you eligible to take a residency on one of the Dental Specialties, not Medical Specialties. Becoming a Dentist does not make you eligible for a Plastic Surgery Residency the same way becoming a Physician does not make you eligible for an Orthodontic Residency.

Botox ?: Botox is a very, very, very small part of cosmetic dentistry, and does not address any problems associated with the teeth themselves. You would not want the periodontist who is performing your gum grafts to give you a face lift, would you ?

Never: Plastic and cosmetic surgery should only be done by people who are trained in the specialty and board-certified in plastic surgery. Beware the "weekend warrior" whose only training is taking a weekend course.