I think you could add it easily to melt and pour.
Maybe make a tea with it and discount your lye to so much then after lye is desolved and cool add your neem powder. Maybe just guess how much and keep a note of it.
I've made some neem oil soap - it smells icky but is good for acne (menopause is SO much fun). There is a very similar product called karanja that has a milder scent.
I've not worked with the powder tho I do have some somewhere.