Modification Making a BJD taller

If the legs have a swivel joint towards the hip area, you could add a segment between the hip and thigh parts. Making the legs longer might bring the body out of proportion and make the arms appear too short, however, so I'd trace the body on paper and try adding length to that first to see how it looks - it might also be worth adding a little to both leg parts and also the torso (if it's 2-part situation) to even it out.

 

If you're not planning on having the doll show their legs, you could try sculpting an extender piece, that would not be permanently attached to the doll but just an option you could take off. They make them for 1:6 action figures, they go on the lower part of shins. As can be seen on this photo right here. Hope that helps, and good luck! (:

 

This is a good idea - you could probably make pieces similar to this yourself.

It's happened with BJD too - for example, some Doll Chateau bodies come with extra joints you can either use or take off.