I created an iPhone application for the App Store.
I hear that some things are not allowed in the App Store. For example you can’t copy existing iPhone functionality.
I didn’t find information about these App Store restrictions.
A friend tell me that I can’t develop my own UITableViewCellAccessoryXXX (I mean something different than the next image) because it’s not allow by Apple. Is that true?

There is no set list of what Apple does or doesn’t allow beyond the high level items in the developer agreement you signed when you created your account to download the SDK. The only way to really tell about these things is to watch from reports online from other developers to see what apps are being rejected (and why). From a technical standpoint, if you can do it with the SDK without using any private frameworks (i.e. if it’s in the header files that are part of the SDK), then you should be in pretty good shape (as far as technical rejections go).
As far as the accessory image, UITableViewCells should allow you to set the accessoryView property to whatever view you would like. As the developer agreement states, Apple has the right to reject your application at any time for any reason – but a custom accessory view shouldn’t be a problem. Keep in mind, you may need to read over this thread if you don’t use a default accessoryView:
Using a custom image for a UITableViewCell's accessoryView and having it respond to UITableViewDelegate