So there is a place near me called buff city soap. They make your laundry soap on site and add whatever fragrance you like. I have been making my own laundry soap(Heavy lye coconut oil only soap), borax, washing soda, citric acid, oxiclean. But I never add any fragrance because to me the coconut oil soap gives a fresh clean smell by itself. Wouldn't adding the FO after the fact introduce oil to your clothing? Which, would be the reverse of what you would like?