Using sproxel and the converter you can basically create models in xml format that can be used to add new weapons and replace some other content in RWR.
Currently the weapon system has the most versatile support for getting in new stuff in the game. Basically you create and convert a weapon model and copy it to media\models, then modify media\models\weapon_specifications.xml by adding another <weapon> block, e.g.
Code:
<weapon>
<specification
retrigger_time="-1.0"
accuracy_factor="0.6"
sustained_fire_grow_step="2.0"
sustained_fire_diminish_rate="0.6"
magazine_size="8"
can_shoot_standing="1"
suppressed="0"
name="Mossberg 500"
class="1" />
<animation key="recoil" ref="27" />
<animation key="cycle" ref="28" />
<sound key="fire" fileref="mossberg500_fire.wav" />
<sound key="cycle" fileref="mossberg500_cycle.wav" />
<sound key="reload_one" fileref="mossberg500_reload.wav" />
<model filename="media\models\shotgun.xml" />
<hud_icon filename="hud_mossberg.png" />
<commonness value="0.2" />
<projectiles_per_shot value="5" />
<weak_hand_hold offset="0.2" />
</weapon>
and change it to refer to the newly made xml-model file with the <model filename="media\models\shotgun.xml" /> tag.
There's a handful of other models based on voxels that can be modified by directly replacing the old file, like knife, grenade and crate debris. The soldier models can't be completely made with sproxel however, as sproxel doesn't provide a way to bind the model to a skeleton. The upcoming RWR voxel editor will help there.