You could try to upload the files via the REST API (see POST /com/vmware/content/library/item/updatesession/file/id:{update_session_id}?~action=add)
But I'm not sure if that would work for a Template.
Worst case, you can try to convert the template to an OVA, and then upload the file
You might also want to have a look at the New-VMTX function in William's ContentLibrary module (but be aware, it's for VMC only).
And there is also an idea "Ability to upload files into Library items" that is planned, but no target date is mentioned (yet).