You can do the import by giving each user a BNS subdomain as a username (see the subdomain registrar). This is what we use when we onboard new users – i.e. your username ends in
.id.blockstack because it was registered via our subdomain registrar for the BNS name
id.blockstack. You can do up to 150 users per Bitcoin transaction.
We migrated all Onename users into Blockstack this way. @aaron might have a script somewhere that can help you facilitate this. However, your users will need to generate a 12-word seed phrase (a private key) and an ID-address so you can sign users up without having to know their private keys.