Serendipity in action. Read this: Rome Total ZOR mod.

The mod does all the export_descr_unit, descr_model_battle and unit card work you'll need.

Just make sure you retain your original export_descr_buildings file and use that in place of the one in the ZOR mod. The game should function the same as it always does, but you'll be able to bribe armies as you want (with perhaps the exception of peasants and a few slave only units). The mod will ensure that you have appropriate (though not always identical) skins and unit cards for the bribed units.

There's a solution to the Custom Battles issue in the first post in that thread as well.

Enjoy!