To answer your question I would have to know what type of soap you are trying to make.
Are you looking to add fruit to melt and pour base, cp soap, hp soap or milled (rebatched) soap?
I've made many fruit soaps and did not add any preservatives. This is something I exclusively do with hot process soap. After the soap is cooked I have added about 2 tbsp per pound of oil of a fruit pureed in a blender with a small amount of water and/or lime juice. I have used banana, strawberries, kiwi (I recommend de-seeding them 'cause the seeds are very sharp), mango, cucumbers, pumpkin puree and applesauce.
I wouldn't recommend this for a M&P soap base but the PH of lye soap will keep anything from growing in it as long as you don't have big chunks of fruit in there. I never found any mold growing on a single bar of my fruit soaps.
You may also be able to do this with rebatched soap but as far a cold process soap is concerned I don't know. A more experienced cp soaper may be able to chime in here.