Good question! You can DIY Essential Oils from plants but it is labor intensive and requires massive amounts of botanicals to make enough fragrance to scent soap. Check YouTube for more info,
The only thing I can remember making, years ago, was an HP soap made with condensed milk. It carmelized and the soap smelled delicious. If you have access to vanilla beans, I think there may be a way to fragrance your batch by infusing them in oil, similar to the technique for making Carrot Tissue Oil. Not sure.
My oatmeal, milk, and honey soap doesn't have any fragrances in it because I make it for someone who has scent sensitivity but it ends up having a sweet caramel scent to it from the honey and milk I add.
You would need to be able to distill or extract fragrances - you can steam distill things like rosemary or lavender using a still setup - however you would't be able to sell this soap as you could not be certain of the purity pof the essential oils you produce. - be careful too as essential oils have allergens and should be handled carefully.