I'm new to ubuntu and I wanted to know if I could

Kbuntu will completely overwirte ubuntu. you could try though to make another partition to install it to. kbuntu is the one with the KDE inerface so it will be totally different the regualr gnome ubuntu.

Ubuntu is just a visual interface over Debian. The DE used on ubuntu is GNOME,
Uubuntu is just a visual interface over Debian. The DE used on Kubuntu is KDE.

so, You can Run only one at a time, but can keep both on system. Installing KDE via synaptic wont change the setup. You can frequently switch both.

Please see Ubuntu or Kubuntu forum for details or ask here, i ll provide link.