Ok, I see.
In that case, first get the current list, then add the user and then call the method.
$HostAccess = Get-View -Id $vmhost.ExtensionData.ConfigManager.HostAccessManager
$currentUsers = $HostAcces.QueryLockdownExceptions()
$newUsers = $currentUsers + $username
$HostAccess.UpdateLockdownExceptions($newUsers)
