You'll have to edit /etc/inetd.conf manually and add 
lines like the following:

  pop3 stream tcp  nowait root ${PREFIX}/sbin/spop3d spop3d
  pop3 stream tcp6 nowait root ${PREFIX}/sbin/spop3d spop3d

The second line is only necessary if you want to use IPv6
support.

An optional global configuration file may be used in ${SYSCONFDIR}
called spop3d.conf.

See spop3d(8) and spop3d.conf(5) for available options.