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.
If the recipe calls for oil, butter, or any other fat, then you can try to scale the other quantities to match it.
If not, there's still a chance you can salvage it. The oil doesn't actually chemically react with the mix at all, it just "coats" it; in fact, most cake recipes use some amount of fat in order to slow gluten formation (i.e. stop the cake from getting tough and chewy).
However, if the recipe called for 1 1/4 cups water and you used the same quantity of oil instead, then that is going to be way too much oil for a single serving.
What I would do is double or maybe even triple the cake mix, make sure it's very well mixed with the oil, then add however much water you needed in the first place.
If you realized your mistake early (i.e. if the original recipe called for a lot more than 1 1/4 cups of water) then you might be able to just proceed as normal and end up with a very moist cake, but more likely you'll still need to add some more cake mix (and the corresponding amount of water) to compensate. I'd still double it, because otherwise you'll be stuck trying to figure out how to use the leftover mix.
Don't just add flour; the amount of water that the recipe calls for is proportional to all of the other ingredients in the mix, and it's virtually impossible for you to know how much extra water you'll need to compensate for the added flour. Besides, you'd only end up with a bland, flavourless cake if you did that.
If none of these options appeal to you then I'm afraid it's destined for the trash, unless you want to prepare your own cake mix from scratch and combine the two recipes.
Best Answer
Unfortunately, the box of cake mix has much more than just flour in it (dried milk, sugar, leaveners, flavoring etc...)... and it may not even use cake flour.
Your best bet will be to buy a second box of the same exact cake mix and double the remaining ingredients as well, so if the came mix calls for 1/2 cup water and an egg, you'll need to add a second egg.
Now, it's possible to come up with a list of ingredients you will need to add to save yourself a trip to the store but we'd probably need to know which cake mix you used to start with.