What to do w/ people who signed up / login via Twitter in the future? (urgent)

I'm not willing to pay Twitter to use their API.

can I create a new "free" app on Twitter and hopefully use that for people to continue to login to my site?

​​​​​​​Additionally 
Is there a way to transform users who used Twitter to sign up / login into 'regular' users w/ username / password to login?

Is it enough if users have a password in the user profile settings?

What would be the easiest way to message/export all email addresses for users who signed in using Twitter? 

in addition to ideally exporting all users who used Twitter in order to email them, I'd love if I could create a module that pops up when people log in using their Twitter to let them know about setting up a password to continue using their account after the API is done .. How can I do that?

 
I imagine this to be rather time sensitive since all API use will be terminated on April 29th . 

ty,
Lars

 

I'd like to know if a NEW FREE app will break the current registration / login function?