DDR2 memory controller implementation ...

Yes, it is possible to use HPC or HPC II without NIOS. You will just need to add a small state machine to feed that data in and out of the interface. If you are reading and writing to the DDR at the same time, your state machine will get a little complex. However, if you write data, then read it, the state machine is a lot simpler. Download the doc of HPC and look at it. It is not too hard to understand. Unfortunately, most of the documentation out there is related to NIOS/DDR usage.