Can I use someone else s product in my own and sell it?

Yes, it is legal and millions of companies do that regularly. You can't imagine that every company has its own raw materials mines or fields for all ingredients for their ultimate product. You just need to steer away from making any sort of false claims, like "contains real Hershey® chocolate", if it's not true.

If you buy a Ford pickup truck, does it need to have Ford brand tires on it? Ford brand battery or audio system? Use Ford brand gasoline?

If you're only adding one more ingredient, then no.

Not legal to repackage and sell the mix. Not sure how you would possibly profit, anyhoo; seems like you would lose money in the long run. Now if you want to use the mix to create an actual dish in a restaurant, that is your call.