passwd backend choosing based on userid

Are there any chance to have passwd backend based on userid?
In order to do this, I use the following in passwd/main.php
if (strpos($userid,"@")) {
    $backend_key="sql";
} else {
    $backend_key="smbldap";
}
but I wonder if this could be done "in a right way", i.e not modifying main.php?
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