Creating User Accounts with our own naming convention and External Identification number questoin

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Hi-

My questions are two folds.  Appreciate any input you can provide:

1.  When auto provisioning users in HCM & ERP cloud, under Security Console > Administration, it gives you the option to create users either by email address, FirstName.LastName, FLastName or person.  Suppose if we want to auto provision the user accounts with our own naming convention, is this a possibility and how would you go about doing it?

2.  For every person we create in the system, we want to associate an External identification number from our Campus Solutions system. We are hoping to leverage APIs to do this update, however, i understand there is no delivered way to pull up an employee by their  External Identification numbers in the system- is this correct?