Or script the auto-add.
Code:
calibredb add --with-library=/path/to/library/ --tags="tag1, tag2" --recurse /path/to/auto-add/
rm -r /path/to/auto-add/*
(paths on windows will look slightly different, and the "rm" command must be replaced with a valid "del" command.)
And run that regularly, or in a scheduled task/cron job.