Has anyone added fragrance oils to natural, unscented laundry detergent?

I like my natural, phosphate-free laundry liquid, which is free of dyes & perfumes. However, I would like a slight natural fragrance (perhaps lavender), but don't want to pay 3x the price for a specialty detergent. Others have fragrance oils in them, but the idea of adding oil to my laundry seems counter-productive. Is there any other natural way to add a scent to laundry detergent? (I'm thinking lavender water may work, but it's not that strong.)