Add user as term store administrators from powershell

To be able to import the data into the term store, the user has to have the permission to do it, which is set in the "Managed metadata web service" service application.

Also it can be easily done in power shell as such,

$taxonomySession = Get-SPTaxonomySession -Site "http://localhost"
$termStore = $taxonomySession.TermStores["Managed Metadata Web Service Proxy"]
$termStore.AddTermStoreAdministrator("administrator")
$termStore.CommitAll()