You need to get the existing job with all its backup objects first and then operate a little on them to make it work. I described all the steps as comments in the example below:
# Get existing NAS Backup Job, all its objects and create a new array out of range of backed up NAS servers
$nasJob = Get-VBRNASBackupJob
$jobObjects = $nasJob.BackupObject
$nasJobServers = @()
foreach ($object in $jobObjects) {
$nasJobServers += $object.Server
}
# Get existing NAS server you want to add to the job and add it to the array of NAS servers you obtained from the existing job
$nasServer = Get-VBRNASServer -Name '*localhost*'
$nasJobServers += $nasServer
# Construct a new NAS Backup Job Objects array out of the array of NAS servers you created above
$newNasObjects = @()
foreach ($server in $nasJobServers) {
$nasObject = New-VBRNASBackupJobObject -Server $server -Path $server.Path
$newNasObjects += $nasObject
}
# Pass the array of created NAS Backup Objects to the NAS Backup Job
Set-VBRNASBackupJob -Job $nasJob -BackupObject $newNasObjects