I'm not sure its really possible at this stage (not without a lot of knowledge of Unity and 3d modelling)
Its not like X1 where you could just copy some sprites from one folder to another. 99% of the assets for X2 are contained in Unity asset bundles. It is possible to alter the bundles and replace assets. However, the Xenonaut armour is going to be its own 3d model with its own texture and animations. I'd imagine you can't just switch the models as it's likely the local forces have their own 3d models with their own textures and animations.
If the local forces and the Xenonaut starter armour share the same model then it might be possible to swap the textures but that would require finding the texture in the 370 odd asset bundles, extracting them, swapping them and then saving them back to the bundle.