How to add a font in localhost:/System/Library/Fonts?

I'm not sure but I think localhost:/Library/Fonts is for everyone and localhost:/Users/my_name/Library/Fonts is for one user. What is for localhost:/System/Library/Fonts?..
if you are the only user then put it in the fonts directory that starts with users/your_name/ ...

If others are using the system as well and you want them to use the fonts as well put the in the fonts folder that is in the /library :)

/System/Library/Fonts/ is for the fonts Apple shipped with the OS. You dont add anything there. Or anywhere under /System/ for that matter.
The /System folder is not really for the normal or even the admin to mess with. You could really screw-up your system if you don't know what you are doing. Personally I look around but I try not to mess with anything directly.