Integration of a Noc in NIOS system

If speed is not a huge issue I would write a simple block that acts as a wrapper. Maybe with a small buffer to make sure of no loss.

This is very easy, if you know vhdl or verilog. You write your little block with an interface to your NoC component and one interface towards avalon. This should have a clock, reset, address, read, read_data, write, write_data and whatever else you need and then you go into SOPC builder and add your own component.