Hi and welcome to Sabayon-Forum,
I'm using Xfce too.
How to activate the keypad automatically at boot knowing that numlockx is installed ?
I don't know how I did this. It's a long time since I needed to do that. Doesn't remember Xfce this on it's own? Perhaps it doesn' occur to me because I'm using parts of the gnome settings.
How to create a new user (adduser doesn't work --> useradd ??) ?
Yes there is no graphical way to do this in Xfce and I'm perfectly fine with this. You don't do this in every session so a commandline-tool is fine for me. As you mentioned the command is useradd. You need to run it as root. The password for a new user can be set with passwd. For more information there are allways the man-pages.
How to correct time ?
I only needed to correct time once and I think I did it with hwclock. But I cant say for sure that it will do what you want.