Is it possible to add further layers of prepreg to a cured part and re-bag & bake?

Will the resins bond ok to the cured parts - I'm assuming the cured part will need keying up?

Thanks in advance

Yes.  It is just like with wet-layup.  You will need to prepare the surface for secondary bonding by sanding and/or peel ply removal - whatever it is you do.  Most times people will use a resin film/glue film/film adhesive to add a little extra (often toughened) resin to the bondline.  If you are using high resin content pre-pregs or many layers (more resin) it may be fine.  I'd suggest either heat-gunning or a warm debulk to make sure the resin on the first ply down soaks into the cured carbon and there isn't trapped air.