Is there any way to manually add cores to RetroArch on muOS?

Hey y'all, just got a RG40XXV recently. I installed muOS, a bunch of games have been added, and I've themed it out. When I was adding my games recently I threw over a few GameCube and Nintendo DS titles, figuring I could install the necessary cores through RA. I noticed when I would try to go to the Online Updates, core downloader does nothing. Is there any new link I should be adding to the Buildbot Core URL? Or a way to manually add cores through folders and stuff? Thanks in advance!!