In my company there are a lot of computers with different IPs and netmasks and I need to access them. I haven't found a way how to do it with a GUI, since a lot of things can be done via GUI in Ubuntu.
So, for you who are interested in the solution, here it is:
sudo vi /etc/network/interfaces
and edit the file to get something like:
auto eth0 eth0:1 eth0:2
iface eth0 inet static
iface eth0:1 inet static
iface eth0:2 inet static
Now I have three IPs (192.168.0.2, 10.10.33.10 and 10.10.50.10).