Bobby,

         There are a couple of variables to involved in answering your question.

         Windows 10 has an app separate from, but closely allied with, the mail app called "People."  It acts as the address book.

         I use Gmail and access it via IMAP on several different e-mail clients, including the Win10 mail app.  The Win10 mail app has automatically synchronized my existing contacts in my gmail account that I use as my Microsoft account ID to my People app as well as those from the specific gmail account that I'm actually accessing via the mail app.

         If you have always created your contacts in your local address book under Thunderbird, which is quite likely, you can always export those contacts then import them via the web interface for the e-mail account you've been using.  From there the People app will synchronize them automatically for any account that is in the mail & people apps.  I have yet to find good documentation on doing an import from, for instance, a comma separated list or excel spreadsheet of e-mail addresses straight into the People app.  It seems to presume you'll be using a modern e-mail client that synchronizes both messages and contacts with the server.

Brian
--