Ok, so I was wondering if this very specific app that I need exists:

Here's my problem. I have one of these that uses the iPhone's output to generate a radio wave and thus listen to the iPhone in my car. Now that works great. My problem is that I don't have a CD player in my car either. So I thought that I could just get a diskman (yeah, remember those?) and listen to it the same way I listen to my iPhone. I won't have access to the CDs before I actually have to listen to them, so it'll be impossible for me to just import them into my iPhone using iTunes. The problem is that the diskman doesn't have the apple jack as an output, it has the good old mini-plug audio jack. So, I was wondering if it is possible to connect the diskman to the iPhone using the iPhone's mini-plug jack and having an app that takes this signal and outputs it to the FM transmitter. Do you think there's an app for that? Do you think that's even possible?

Thanks a lot for your help! I'll really appreciate anything that you can contribute

PS: in case you're wondering, my iPhone is a 3GS, completely jailbroken and running on iOS 4.0.1