There are several challenges to this. The current library only accepts from SdCard. Second one needs a conduit between Raspberry and Arduino.
Serial port would be the most obvious, then SPI, the I2C. Where as throughput is a major concern along with building a layer to transport the stream over the selected interface.
Where there is a fork that I have not yet integrated into the original library that is a Internet Radio Player. It accepts stream over Ethernet. It has the added cost of the Ethernet Shield. https://github.com/gallegojm/Sparkfun-MP3-Player-Shield-Arduino-Library/tree/Due. Where it is originally for an Due. But should work for an UNO.
That all said I would recommend that you look at the following Ada Fruit App Note on Raspberry MP3 player as to not need the Arduino.
