Giving external input to Triple Speed Ethernet

You can use whatever RAM you like (on-chip, off-chip, SRAM, SDRAM, etc.).

The RAM is there so that software (by default iniche) has storage for Ethernet frames. The SGDMA controllers transfer the frames between RAM and Avalon-ST (for the TSE ports).

It sounds like you need to change the memory controller to match whatever memory you have in your hardware design.