I don't know any graphical interface to add and edit users for lxde. But for xfce there is one, so maybe you can install xfce.
It is which actually belongs to gnome and will be installed with
sudo apt-get install xubuntu-desktop
so it should work if you 'only' install that one. I think that, as long as you don't run gnome, it will not slow down your computer, it only occupies space on the HDD.
sudo apt-get install gnome-system-tools
And finally, I have recently installed a desktop system with xfce, added lxde and added a user. That user showed up at login after using users-admin to create it.