Nope, you can not extract the password. In other words you can not re-engineer it.
What you normally would do is let the user enter the password a second time and, when you don’t know the password ‘yourself’. You would encrypt the second attempt and compare it against the one in the database. For ways to do that you can have a look at the encryptor plugin.
Another way to do it is use your own dialog (this is how I do it). Let the user type in the 2 passwords compare them and store them afterwards…