It is impossible. Additional FTP accounts - is the restricted aliases for domain FTP user. As far as domain FTP user can not exit outside of domain www-root, additional account also unable to pass restriction.
However, U can try to create directory inside domain's httpdocs and associate it with external path via mount command, any like
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mount --bind /data/ftp_users/$EXTERNAL_DIR_NAME /var/www/vhosts/$DOMAIN_NAME/httpdocs/$LOCAL_DIR_NAME
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after it set home for additional ftp user as /httpdocs/$LOCAL_DIR_NAME.
Note: not checked, just thinking