FIFO based on DDR

It can be done. I can't share any code because it's proprietary but yes you can do a fifo. I suggest putting small dc fifo's on the data in and data out ports of your fifo code to handle ddr's bursty nature (when you want max throughput) and if you need to transfer clock domains. Also, look at some single clock fifo examples on the web to get ideas on implementing the controlling state machine.