I'm currently running an imac, and am very interested in all the new GPU renderers coming to the market. Octane in particular has caught my eye

My imac does support changing graphics cards unfortunately, so i'm not very technical and was wondering if anyone know if it is possible to hook an additional graphics card to a mac (or pc) as an external device? like an external harddrive i suppose...
any ideas? I'm happy to pay for something like this to use octane, but buying a whole new computer? well...