You are not able to get the passwords out of the hashes.
If you want existing user to be able to log in, you will have to implement the current logic in your project.
End User was so kind to look it up for you:
$password = md5(md5($password).$salt);
All you need to do is:
Have access to the 'old' user-data (You`ll need the username, password, salt Fields from {$tablepre}members).
When someone tries to log in, check if that login exists.
if it does, get the salt and password-hash to that login out of the database.
check if
$database_password_hash == md5(md5($user_entered_password).$database_salt)