Aside from getting a much better photo organizer editor than Photos (hint), try some software dedicated to geocoding.
I'd suggest HoudahGeo. You can add geolocation in several ways. First, you add files...and this might be tricky because of Photos. With iPhoto, Aperture and Lightroom, HoudahGeo could look inside their libraries to find photos. Not so, yet at least, in Photos. But if your photos are referenced, or if they aren't yet copied into Photos (managed), then HoudahGeo processes them. You can add geotags from HoudahGeo's map(s), from GPX logs, or from the bar code used by gps4cam, a terrific app that runs on your iPhone. The data then gets written in various ways to your photos, so upon import to Photos the geotags are there.
Again, the problem with Photos (one of anyway) is that if you manage (i.e. copy) your photos into the Photos library at the present time they are inaccessible to other applications. Even if you reference photos, you'll probably have to re-import.
I'd suggest you take the hint and use something beside Photos.