You can only expand a single property via the Select-Object cmdlet. Try this:
Get-MgUser -All -Property DisplayName,onPremisesExtensionAttributes,UserPrincipalName, Department, Division, City, Country -Expand Manager | Select-Object DisplayName,UserPrincipalName, Department, Division, City, Country, Manager -ExpandProperty onPremisesExtensionAttributes | select DisplayName,UserPrincipalName, Department, Division, City, Country,ExtensionAttribute*, @{Name = 'Manager'; Expression = {$_.Manager.AdditionalProperties.displayName}} | Export-CSV -nti file.csv