Can i buy custom frames and have my doctor place the lenses in afterwards?

Yes, but . . .: Its' sort of like bringing your own steak and asking the restaurant to cook it for you. Most optometrists and ophthalmologists that refract and diagnose vision also sell glasses and contact lenses, including many custom frames from many "brand" sources. If they don't carry certain "names" they can usually get them for you. What you are asking is technically possible, but in poor taste. Ask them.