I don't think there's a way to set static IPs using the built-in system imaging tools. There are, however, a couple of ways to get there:
1) Write a startup script to set the computer's IP address (probably using the
networksetup command), and add that to your image creation workflow.
2) Switch to DeployStudio for the client imaging step, as it has a workflow step for this built in. Note that DeployStudio can use a master image created with Apple's System Image Utility, you just have to extract that from the .nbi image: open the .nbi folder, mount the .dmg inside that, then look inside that image for System/Installation/Packages/System.dmg -- that's the master image you need.
Finally, yes it's possible to mount the image inside the .nbi and make changes, but there's no way to "update" it based on a changed workflow -- it's just a volume, and you make changes in it like any other volume. Also, it's only the netboot image that's editable, not the master image inside (the one that's actually going to be cloned to the target computer).