This is not a supported modification to power CMS.
you would need to edit the super admin password stored wa_setings table to the SHA1 string.
you would need to edit the admin_cms/users_login.php, add the following code at line 1 to convert the password that is entered to an SHA1 string for login:
$_POST["Password"] = sha1($_POST["Password"]);
similar code would need to be added to the admin_cms/users_insert.php pages as well so that new users that are created will have the password stored in SHA1 format. the password form element on that page is named "password" so the code above would need to be altered slightly.
and code would need to be added to the users_update.php page to compare the submitted password with the one stored in the database to see if it is being changed and if so, convert the new password to the SHA1 format.
If you need assistance making this change, we can help you in a premiere support appointment.