Add values from feed into the users database table
Your best bet is to use HEADLESS Drupal. This means using Drupal as a backend, but with the front end being entirely separate to that. You do this by creating a REST server in Drupal, through which you can perform various actions, such as registering and logging in users.

If you are using (or will use) Drupal 7, a server can be created through the Services module. If you are doing it in Drupal 8, it is part of core.

You should set up your system to use OAuth2 authentication. This requires the OAuth2 server module, though I'm not sure what is necessary for Drupal 8.

With the REST server set up, you can create a form on whatever site you want, and you will then register the user through the REST server. Users can be authenticated (logged in) the same way.