Crider83

I haven't heard of one.

It should be straight forward to do yourself. Though as I haven't done it myself, this is only what I think will work.

The defense structure folders you are looking for are here.

\dgdata\units\ug\b

Open up the relevant folder for a specific structure and you'll see the unit.bp files.
eg: UGBDefense01_unit.bp

Since towers do not grant XP, You'll need to add a section like

Experience = {
        Radius = 30,
        Value = 20,
    },

and fill in whatever values you think are appropriate

There is of course more work than this to build a mod... but the other steps are the basics like creating and filling in the mod_info.lua file etc... covered in other posts.

Don't modify your game files directly, that will not let you play in multiplayer with anyone else and may break the game completely if you do something wrong.

Here's a mod that adds XP and gold for tower kills:
300 for archer towers
500 for ToLs
1000 for forts

http://code.google.com/p/nubletsupreme/downloads/detail?name=TowerRewards.zip

Extract directly into your mods folder.