How to add external conduit simulation model when doing NIOS II simulation

When run the NIOS II simulation in modelsim-intel, how to add external conduit simulation model when doing NIOS II simulation?

The simulation setup and method I used is based on example AN351.

For example, NIOS II -> Avalon_mm SPI to external module.

where can I define external SPI simulation module? I don't see anywhere I can modify test bench files like the simulation without NIOS design.