Add term with specified guid to termstore
add-pssnapin microsoft.sharepoint.powershell
$siteUrl="https://buyingportal.dcs.avd.com/gbp/portal"
$taxonomySession=Get-sptaxonomySession -site $siteUrl
$termstore=$taxonomySession.DefaultSiteCollectionTermStore
$taxonomySession.Termstores
$avdGroup=$termstore.Groups["avd Group"]
#$avdGroup
$material=$avdGroup.Termsets["Material"]
#$material
$targetSys=$material.Terms["TCA010"]
#$plant=$targetSys.Terms["RXN"]
#$plant
$plant=$targetSys.Terms["LO1"]
$matGroup=$plant.CreateTerm("I_FAC_CAT", 1033, "6b98a872-3120-4130-b76e-47a3089fb0ee")
$termstore.commitAll()
#6b98a872-3120-4130-b76e-47a3089fb0ee
$matGroup