Custom Spi Interface controlled by HPS in Arria 10

I'm not sure I understand your question. You can configure one or more of the HPS SPI controllers to send the SPI signals over to the FPGA section instead of HPS pins. In that case your FPGA design must connect those signals to FPGA pins and it doesn't go through the HPS to FPGA bridge.

Alternatively you can put an Avalon SPI master component in the FPGA and connect it to the HPS to FPGA bridge. The SPI signals then need to be connected to FPGA pins and can't be shared with HPS GPIOs.

Both solutions are fine but you must just remember that the HPS SPI controller and the Avalon SPI controller don't have the same software interface and will require different software drivers.