Les, here is the documentation for adding a custom event to an object (which would be the customer or user object you could attach to either). https://docs.xperience.io/k11/custom-development/handling-global-events/handling-object-events. This would execute any time a user is created for anywhere if you go this route.
I don't quite know if you have portal engine or MVC, but I'm guessing portal engine because of V11. For that you might need to clone the ecommerce module or web part for the step you have, and write code into the .ascx file where it makes the most sense, but that is a lot of work. I would need to know more about your setup to say for sure.
No, it is not possible to read a user's password from the DB (in plain text anyways), that would be a security risk. You can set it (write to it) as much as you want though.