Can you tell me if braces help extend narrow palates?

No: Braces move teeth, period. To change the configuration of bone in a child you need an expander, in adults an expander and surgery. Please see a fully trained orthodontic specialist for examination and care.

During growth yes: During growth (males up until age 15 and females 12), skeletal expansion of palate is possible. But beyond those ages (with a great degree of individual variation), the skeletal structure of the upper jaw won't change unless with jaw surgery. The arch form can be modified though to some extent as teeth can be pushed out a bit in narrow arches with braces, with limitations and potential relapse