There is no easy way.
FPGA configuration can initialize the internal RAM, but not external one. Some kind of agent is needed to write to external RAM.
When the design has a NIOS processor, there is a debug module that can receive data from the PC and write it to external RAM. Others designs, such as the Control Panel in some developer kits, can offer similar functionality.
With your own designs and without a CPU, you would need to provide such an agent, otherwise it is not possible.
If your board has SRAM, you could use a NIOS design or Control Panel to initialize the SRAM content. Then you can switch to your own FPGA design and the data would still be there. This works on static RAM only, DRAM needs refresh cycles so DRAM content is usually lost when reconfiguring the FPGA.
