I would like to add a custom iAd to my app. I don't want to release it to any other apps. I just want it to be in my app, no one else's. I also want to have other people's ads, too.
Is this possible?
I think you are looking at two possibilities here:
First option: you pay money to distribute your advertisement through iAd and maybe somehow limit it to be displayed only in your app. (would not suggest)
OR you could (and should) just hide the iAd from time to time and display your own advertisement instead. That way you don´t need to rely on Apple to show your ad and don´t have to pay anything. This way you may also be able to show your ad if the iAd is unavailable due to flight mode or something similar, IF the ad is locally available in your app.
You may also have a look at alternatives for iAd, maybe they suit your requirements better.