Plugin that Adds Tags from a List to Every Post

Can someone suggest a plugin that will tag every post made with a certain # of tags from a list?

I have a phpBay site that revolves around a certain niche. I also have a plugin that converts searches into posts the result of which is about 5 new posts automatically generated per day.

In this niche I have about 20 keywords that relate to it and would like to have these added automatically as tags anytime a new post is created (by visitors to my site as they search).

Ideally, I could plug in a list of 20 tags and whenever a new post is made, the plugin would randomly grab 5 or 6 or however many I specify from the list and apply them to that post.

Right now I have a bunch of posts that have no tags and I have to go in and manually add them…which is getting old fast.