Automatic wordpress tags?

I have a wordpress blog with automatic posting plugin that fetches content from other sites.
I would like to add automatic tags to all the thousands of posts, but i have not found any plugin jet that does the job.
By tags i dont mean the meta tag but wordpress tags. I have one competitors site that uses tags and gets alot more traffic due it.
Adding all the tags manually seem like endless effort.

Maybe i can fetch my competitors tags and then add those in tags plugin.
Or there is some plugin that im not aware of that does tags automaticly or even better plugin that keeps in mind actual keywords?

Any help welcome, thanks!