So, I know you can add on to regions in the 'Modify Properties' part of a region, but is there a way to do it directly on the Terrain Editor? It'd be so much easier.

No, there isn't a way to directly add region shapes, which is annoying as hell. You can, however, move around the shapes that you make directly in the Terrain Editor, which makes positioning easier. Just click the list of region shapes and the region will turn green, and you can position shapes to your liking. Resizing and making new shapes seemingly can only be done in the Modify Properties window, however.

You probably already knew everything I said, but I thought I would help confirm it.

Also, regions cannot handle more than 50 shapes (0-49). It will look like it will, but when you test the map, shape 51+ won't work, and when you reopen the editor, they will be gone.