Yes, it is. All the themes that are to be used by all users in a system are placed in the /usr/share/themes folder as subfolders. So, the only thing you have to do is copy the theme folder in there as root. The folder name will be the theme name you see in Appearance.
In case you don't see it in Appearance, make sure the correct permissions are given in that theme folder and mainly that it's readable by others.
Alternatively, if you want the themes you extract or copy to be available to your account only, you can create a folder named .themes
in your home folder and copy the theme folders in there. The dot in the beginning of the name is essential. One way to create it is to open a terminal window and type:
mkdir ~/.themes
.