We're trying to design a seamless in-app onboarding process, can anyone suggest a simple onboarding pattern/process of other products that you like?

You can look into Single Sign-On (SSO) and let the employees self-onboard. Permissions can be mapped in the SSO provider and then loaded when the user logs in.

Or you can use a pattern similar to MailChimp. Allow an admin user to either copy/paste the data in an editor that parses columns appropriately. In the following step, the admin user then makes sure the columns are mapped accordingly.