force broadband connection to H3G network

["Nicolo' Chieffo" <nicolo chieffo gmail com>]

Hi, I have an H3G broadband connection and I would like to know if
there is a way to force network-manager to only use H3G network and
not go to national roaming in not covered areas: the roaming traffic
is much more expensive.

I see that in the connection option there is an entry called
"Network", I tried to write "3" or "H3G" but I don't think it works in
this way, since I can't click "apply". Can you help me?
Thanks

Nicolò