Nios to Avalon signals

You could put them into an on-chip ROM using an initialization file. In Qsys there is an on-chip RAM component that can be setup as a ROM. Near the bottom of the RAM GUI the name of the .hex file which should appear in the Quartus top level directory after the system is generated. I prefer to provide my own file which you can specify in the RAM GUI as well and to create the .hex file just tell the RAM GUI where you plan to put the file and create the file using the Quartus memory editor. A small RAM maybe 32-bit wide an a handful of words deep should be very cheap in terms of FPGA resources, if you keep it small it might not even use RAM blocks.

Once your memory is read then Nios can access it if you hook the data master up to it. To access the 32-bit value you can do something like this:

# include "io.h"

# include "system.h"

unsigned int registerA = IORD_32DIRECT(RAM_BASE, 0);

unsigned int registerB = IORD_32DIRECT(RAM_BASE, 4);

unsigned int registerC = IORD_32DIRECT(RAM_BASE, 8);

etc....

That "RAM_BASE" is what your memory should be called in system.h if you named it "ram" in Qsys. Just double check what system.h lists in terms of macro names if the name happens to be different in your system. You could also put this information into an external memory but you'll need a controller for Nios to access it. It's very common to put things like MAC addresses and serial numbers into external flash because you typically need those to be independent per board.