Converting post text into tags?

I have just been tasked with upgrading an old WP site to 2.8. So far so good. Everything upgraded nicely.

The one thing that I’m not figuring out is that in each posts, tags were inserted in the following format:
[tag]Mytag1, Mytag2, Mytag3[/tag]

I believe it was used with one of the 3PD tag plugin, but haven’t found the information about that.

Is there a plugin or an easy way to automatically go through each post and put these tags in the WP tag field? I *could* do it manually, but as there are nearly 500 posts on the site, I’d rather not spend time there.

I can’t seem to find this information on the Codex, in the forum, or through Google either.

Thank you for any and all ideas.