to the furthest stretch of my imagination, i cant contemplate why anyone would use fedora/rpm distro (except a n00b)
Anyway to give some help piva.francesco, you need to find the package that has the useradd command. the problem is that package may need 5 other packages installed to work and so on.
This is a major problem with these kind of distro's
As uman said, switch back to slackware or goto debian. fedora offers nothing special