Manual generation of an nb0 :
Platform builder environment provide two tools for the generation of the nb0 files. The first one Viewbin used to see the properties of the bin file, the second one Cvrtbin used to convert the bin file into an nb0 file using the information provided by viewbin.
So open your OSDesign and go to Build -> Open Release Directory in Build Window, it will show up a command line window setup for your project.
Use : viewbin nk.bin
Image Start = 0x00220000, length = 0x00AC136C
Start address = 0x0023D618
Checking record #72 for potential TOC (ROMOFFSET = 0x80000000)
Found pTOC = 0x80ce02c0
ROMOFFSET = 0x80000000
cvrtbin -r -a 00220000 -l 00ac136c -w 32 nk.bin
Values may vary for your nk.bin as those information are related to the settings of your BSP and OSDesign.