Yes, I agree with Suzanne. Sifting together is a great idea.

I just looked this up to see what my books had to say. Self-rising flour is believed generally to have 1 1/2 t (7 g) baking powder per cup. Apparently one of the things self-rising flour does really well is to evenly disperse the leavener through the flour, so I recommend sifting AP flour and baking powder together a few times or running them through the food processor to thoroughly disperse.

All-purpose flour can be substituted if you add baking powder and salt. For each cup add 1 1/2 teaspoons of baking powder and 1/2 teaspoons of salt and mix together.