Configuration information is stored in an SQLite3 database in the "forward" table. To add a forwarding rule, you simply have to insert it into the database, e.g.
$ sqlite3 /var/lib/sipfwd/sipfwd.db
> INSERT INTO forward (local, remote)
VALUES ('user@example.com', 'sip:account@sipprovider.example.com');
> INSERT INTO forward (local, remote)
VALUES ('sales@example.com', 'sip:166@voipfone.co.uk');
> INSERT INTO forward (local, remote)
VALUES ('time@example.com', 'sip:123@voipfone.co.uk');
This will forward any SIP connections for user@example.com to account@sipprovider.example.com.
Note that the local column has to be specified as username@domain, whereas the remote column has to be specified as a full SIP URI.