1. Download the kernel source files.
2. Locate the place where the inode struct is defined.
3. Add your new field.
4. Recompile your kernel.
Of course this probably will break half of your programs and will be guaranteed to corrupt your file system, but hey, it's open source and you are free to do with it whatever you want.
Sorry, could resist.
Problem with your question is that it's to vague to give a clear answer.
For example. You don't tell us what file-system you're thinking of.
Also you don't tell us why you wan't to add this field. If you would explain the reason for it, maybe someone could direct you to an alternative that is more in line with what you want to do.
If you are serious about going into kernel development you should visit the kernelnewbies site