Best way to expand 5304T

My 5304T with 4 drives in it is nearing the full mark. I know I can replace all 4 drives with larger ones but that seems like a waste of the original drives. Is there a good option to expand my raid 10 array with another enclosure and have it appear as one drive? Im thinking trying to split my plex library over 2 drives would be a bit of a mess. Thanks!