Following commands are used for import & export mailbox in pst file.
By default, the Mailbox Import Export management role isn’t included in any of the built-in role groups, such as the Organization Management role group. To import or export mailbox data, you need to add the Mailbox Import Export management role to a role group.
Following command is for adding assigns the Mailbox Import Export management role to the security group.
New-ManagementRoleAssignment -Name “Import Export_group name” -SecurityGroup “Group name” -Role “Mailbox Import Export”
Following command is for exporting user primary mailbox to a .pst file
New-MailboxExportRequest -Mailbox User -FilePath “c:\\User_Recovered.pst”
Following links are usefull for the same.