Can GAN generate very happy facial expression
Here's a paper by Tom White and its repository studying the specifics of sampling from generative models. Section 3 in the paper, Attribute Vectors, might be of interest for your problem. I think you'll likely have to rely on another dataset to learn the latent code for a smile to use on your own images, but given the results of the author's own work it should be fairly good.

Hope this helps!