You can manually add GPS coordinates to photos either singly, or in a batch.

Select one or more photos in a contact sheet. 

Go to Image>Set GPS Coordinates

GPS coordinates may be entered manually using coordinates or by using the Google Map® interface to search for a location. 

Use  Show on map to display coordinates you've manually entered to see if they're what you expected. Once you're happy with the location, use the Accept button. If you selected a batch of photos taken in the same location, use the Apply to All button to give all selected photos the same coordinates. Use the Apply to Remaining button to set the coordinates on the current preview and the photos that follow it in your selected set of photos.

If you aren't sure of the exact coordinates, you can use the Google Maps interface to search for the location. You can enter points of interest, cities, or full street addresses and then click on the Get GPS button to look up the location and plot it on the map. You can pan the map around and the marker will center itself at the new location, or you can pick up the marker with your mouse and drop it at the desired location. Use the Accept button to copy the location into the GPS Coordinates section of the dialog. You may now apply the location to one or more photos.

Use the arrow buttons to move through your selected photos, setting GPS coordinates as you go, until you have set them all. Use the Close button to dismiss the dialog. If you often shoot in this location, take advantage of the snapshot button to save time in the future. 

If you already have GPS data, you can also import it to save time.