Hi @Kush1 !
TL;DR
Could you make a quick test - mainly, once a user is created in Auth0 as a result of migration from your legacy db, could you assign the user via Auth0 dashboard to the desired organization and test if it will have the needed access?
Unfortunately it will not handle passwords migration.
For already existing users yet not added to any organization, you could call the Management API endpoint to send a maximum of 10 members per organization per request.
For new users I can think of adding a bit of code to your migration script, mainly, once a user is created in Auth0, call the Auth0 Management API to add user to a specific organization.
Please let me know if I can help more!