External tables are data stored outside of BigQuery’s managed storage, in other words, persistent external data.The Temporary tables are tables that exist only for the duration of your query session. So linking a temporary table to an external data source doesn’t make sense because the temporary table would vanish, leaving the external data reference dangling.

I think the best workaround is to create a regular external table, this way, the table definition persists, so you don’t have to recreate it every time.