You can easily add normal Tor bridges to the Tor config file, because it's easy to get a list. But it's not easy to get a list of obfs bridges, because there is no public list.
Which lines do you mean? The ones which TOR Hacker posted? Just replace the username "toranon" with username "ofrester". Note that this will not block all traffic except Tor. It will block all traffic except the traffic from the user account ofrester.
To block all traffic except Tor while still using the pluggable transports TBB you could add a new user account to Ubuntu.
sudo adduser tor-user
Move TBB to the new users home
sudo mv /home/ofrester/tor-browser_en-US /home/tor-user
sudo chown -R tor-user:tor-user /home/tor-user/tor-browser_en-US
Replace the username toranon with tor-user.
iptables -I OUTPUT 1 -m owner --uid-owner tor-user -j ACCEPT
Then you can start Tor from the ofrester account with
sudo -u tor-user ./start-tor-browser
Never log in as user tor-user however. Always log in as ofrester.