crypt without the use of salt

There is a system in PHP that was not developed by me, it uses crypt to encrypt the user's password, like this:

$senha = crypt($senha);

With the PHP update on the server, crypt is asking for salt when creating new users password.

The problem is that there are already more than 10,000 users registered in the system without salt. If I start now to include the salt, I will have problems with the login of the old users, because they have no salt.

What to do?