To enable the Migration administrator account to impersonate users, the following will need to be run in Exchange Management Shell:
New-ManagementRoleAssignment -Role ApplicationImpersonation -User Migration Account
Below is a Microsoft article that explain more about granting impersonation rights.
New-ManagementRoleAssignment -Name:MigrationImpersonationAssignment -Role:ApplicationImpersonation -User:userName -RecipientOrganizationalUnitScope "domain.com/Microsoft Exchange Hosted Organizations/company.com"
For more information on this Exchange 2013 command refer to the technet site: