You need to generate new keys (or use existing other client/username keys) for the new client/username
On the server:
cd easy-rsa . vars ./build-key username
On the client:
Copy username.key, username.crt and ca.crt from server to /etc/openvpn/keys/.
cp -ai /usr/share/doc/openvpn-*/sample-config-files/roadwarrior-client.conf /etc/openvpn/client.conf
chkconfig --level 2345 openvpn on
service openvpn start
On the server:
cd easy-rsa . vars ./build-key username
On the client:
Ideally the client should do some verification on the server key with tls-remote in the whatever.ovpn configuration file.