Adding a Zip Code

I’m wondering if anyone knows a way to automatically add data to a post based on a tag.

For instance, what I’m wanting to do is automatically add a series of zipcodes based on what cities are entered in as tags on a post.

So, if I enter in “Springfield MO” as a tag the post would automatically add 65801, 65803, 65804, 65805, etc… etc… (whatever I define it to do based on that tag)

Or… think of it this way.
If I enter in “car” as a tag, I want “red, blue, green, black, yellow” all added to the post as well.

These things don’t have to be visible… and don’t have to be added IN the tags area… just as searchable text in the post… even if it’s hidden. (it would be best if it were hidden)

This is (obviously) something I know I will have to make in a file somewhere… but, is it possible to do?

Maybe there’s a plug-in out there that adds content to posts based on tags.

And… if not tags… what about by categories??

Thansk!