I have a pineapple-orange cake recipe that is very good, but is from a relative - I didn't develop it. I am curious how to adjust a typical cake recipe or even a box cake mix so that I can add fruit (pineapple, peaches, cherries, apples, oranges, mango, etc...)? Do I need to adjust the baking soda, oil, flour, sugar, etc...? Thanks.
Adding fruit will change the ratio of dry to liquid, so it may require more flour. It would also add some sweetness, so you may want to reduce the sugar a tad. But citrus fruits in particular are acidic, which might affect the leavening. I suspect it would depend on the type of fruit and how it's prepared.
For something simple answering the box cake mix portion of the question, you might want to consider variations on the "dump cake." Fruit, usually canned but sometimes fresh and sometimes mixed with gelatin, is placed under a box cake mix with melted butter or butter and water on top and baked. This allows you to do a variety of fruit flavors with one cake.