Normally, this can't be done. I have written a class which wraps around the usual Storekit functionality and will allow an app to behave (while running on the simulator) as if the Storekit transaction had succeeded.
I have written a blog post about the design decisions behind the code, which can be read here and includes a download link: http://code-evolution.blogspot.com/2010/05/evolved-code-example-1-simplestore-for.html
Or you can just grab the sample project from this location: simplestoreevolved.googlecode.com/files/SimpleStoreEvolved.zip
Enjoy
T.