I've never tried to take a copy of a 1shoppingcart database, so I can only comment in terms of the process one would follow:
You'll need to first determine whether you can even extract that data in the first place.
Then you'll need to massage the data into a form that can be imported into the corresponding Zen Cart database tables. Customer data is spread across three tables: customers, customers_info, and address_book. And address_book data is cross-linked to records in the zones and countries tables in addition to being cross-linked back to the customers table.
So, that will probably involve some significant spreadsheet manipulation tricks before doing the actual import of the data using a suitable program for accessing the raw database.
And all your customers' passwords will need to be reset manually by the customer before they will be able to login. They can of course use the Forgotten Password link on the login page to have a new password emailed to them.
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