Import can be used for adding new “items” to wordpress, but not changing existing ones. If you want to update anything you should use the xml-rpc protocol with the relevant commands https://codex.wordpress.org/XML-RPC_WordPress_API, or if you need it only locally wpcli http://wp-cli.org/ might be even better (never used it for your specific type of change but I assume you can do it with it)
