This is super easy!
Say your recipe requires 650g water and 220g lye (fictitious numbers!). You want to add 320g water (I always add 100g more water than the amount of lye) and you can then add 330g pumpkin puree.
You just need to be sure that you have enough water to dissolve all of the lye. Theoretically you could do so with 220g of water, but I like to be on the safe side and add 100g extra. I do this no matter what the measurements are for my recipe, 100g extra water to the lye amount and then the difference in whatever I'm adding.
I use the above for all food additives EXCEPT avocado! That one I use a superfat discount rather than adjusting the water.
Hope this helps! I've made a few batches of really nice pumpkin poppy seed soap with this method, works very well

If you're using tinned pumpkin you may want to add lye amount plus 200g of water, as it is less liquidy than hand pureed pumpkin! Or maybe add some goats milk to the tinned pumpkin to make it more liquidy.
