By Alexandra Alper and Karen Freifeld Reuters (Adds comments from assistant secretary nominees for Commerce, Treasury) WASHINGTON, Sept 21 (Reuters) – U.S. President Joe Biden's nominee to a post overseeing export policy on China said on Tuesday he sees Chinese telecoms firm Huawei as a national security threat and expects to keep the company on a trade blacklist unless "things change." If confirmed as Under Secretary of Commerce for Industry and Security, former Pentagon official Alan Estevez also pledged to "look at" Honor Device Co – a handset unit spun off of Huawei – to see whether the telecommunications company…