I have done this with moderate success, and I would say it's probably easier with Posette than other figures because of the mapping of the face.

Essentially, the process is as follows:

1. Take a full face photo of your niece.
2. Take a skin texture for Posette.
3. Bring both into Photoshop or similar as different layers.
4. Remove everything in the photo that isn't the face; resize to match the texture.
5. Make the photo layer 50% transparent so you can see how it overlays onto the texture; move and rescale until it fits the face on the texture underneath. Then remove the transparency.
6. Now adjust the hue-saturation-brightness of the underlying texture until it matches the photo.
7. Now flatten the layers and tidy up the edges of the face (a lot of smudging and blurring).

Then use this on a suitably bland and childlike Posette figure.