Well, you would have to make a login with PHP to do that that reads from the wordpress database. It would be really easy to do. Heres an example
if (isset($_POST['submit'])) {
$dbc = mysql_connect('database location here, 'username here', password here');
@mysql_select_db('database you installed wp in');
// username and password sent from signup form
$username=$_POST['username'];
$password = md5($_POST['password']);
$query= "SELECT username, password FROM wp_users WHERE user_login = '$username' AND user_pass = '$password';";
$result=mysql_query($query);
// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row
print "<p>$count</p>";
if($count==1){
// Register $myusername, $mypassword and redirect to file "login_success.php"
$_SESSION['username'] = $username;
header ('Location: admin.php');
exit();
}else {
echo "<font color=\"red\">Wrong Username or Password</font>";
}
}
?>
This will search the wordpress database for a result matching whats in there. So, if you fill in the fields correctly with your own database location, username and password, this code will work. If not, it will tell you that your username and password is wrong. Hope this helps. If you need help, just email me.