I was wondering what an addon mesh is and what could be done with it.
Simply put, could a basic unit body be modeled, and then have armor (forearms, helmet, shin-guards) attached as addon meshes? It would greatly increase the easiness of not having to re-weight edit and all that stuff you have to do when exporting units.

I'm pretty sure you can only have one addon mesh on a specific unit unless you hex edit them on (I'd much prefer weighting over hex editing, hex takes more work). In your case why don't you just make your basic unit body and add the other pieces in XSI? I really don't see why you'd need to use addon meshes instead.

Funny, I was also wondering how I would add one addon mesh to a model. I could not find any tutorials.

The FAQ / Everything You Need Thread Version 2.0 - Attaching a BF1 Cape to a BF2 Character

The name just isn't obvious.

Does anyone know if AnimationAddon works for SWBF2?

I only know that is is possible to add dookus bf1 cape to a bf2 character. Just type animatedaddon to your unit's odf and the name of dookus cape.