Basically I think bless-ing your socket as a Net::SMTP object will work, but have a look at the Net::SMTP->new method for some additional initialization that goes on.

use Net::SMTP;

# create your own socket however you want
my $sock = IO::Socket::INET->new(...);

net_smtp_from_existing_socket($sock)
  or die "couldn't do it, sorry";

sub net_smtp_from_existing_socket {
  my $sock = shift;
  my $hello = shift; # the optional hello message

  bless $sock, 'Net::SMTP';

  $sock->autoflush(1);
  # can set $sock->debug here if you want

  unless ($sock->response() == CMD_OK) {
      $sock->close();
      return undef;
  }

  (${*$sock}{'net_smtp_banner'}) = $sock->message;
  (${*$sock}{'net_smtp_domain'}) = $sock->message =~ /\A\s*(\S+)/;

  unless ($sock->hello($hello || "")) {
    $sock->close();
    return undef;
  }
  $sock;
}
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Alternatively, use Net::SMTP->new to create the socket - the returned object itself is an IO::Socket::INET object.