If you would like to add photos to an existing gallery you must first enter the gallery. You can do this by clicking on its name in the list of galleries.
Once you've entered the gallery you'll see two sets of options for what to do with that gallery on the right. Pay note to the upper set of options where you'll see one labeled Add Photos.
Clicking on the add photos option will give you a choice of several ways to upload your photos. Whichever one you choose, the images you upload will be added to the currently selected gallery.