How can I migrate subscribers usernames and passwords from bigcommerce to magento?
Magento offers an import/export functionality for customers.
You can see this in the backend in System->import/export.
Just export a set of customers and you will see the format you need for import.
Then create a csv file with the same format as the one you just exported.
There might be a problem. If the password in your bigcommerce website are hashed differently from the passwords in Magento you will not be able to keep the passwords.
Also, there might be newsletter subscribers in magento that are not actually customers.
But this can be solved easily.
After importing the customers, you can just insert in the table
newsletter_subscriber the newsletter subscribers from your other project.
The table columns should be straight-forward.