the iphone doesnt really support attaching extra storage to it.
its been a while since i played on one. but if you jailbreak it, you might be able to mount an sftp drive. if you can. you could in theory mount it in a video player app document dir or something like that, so that it would be able to read its content natively. but without a wifi adapter, the pandora tend to have very limited speed, so it would be slow at best.
i know that for windows and mac, there is a usb network driver for the iphone, that basicly let you connect to the iphone using a network interface, but on the usb cable which is much faster. although i dont know if such driver exist for linux, let alone open source.
For both those method, youll need a jailbroken iphone, and i would not recommend it to someone with limited experience with unix.