This requires you to change the model itself. It can be done with the help of bodygroups, like the grunt model that has 4 heads (bodygroup: heads) and 3 weapons (bodygroup: weapons)*. Now when you turn on SmartEdit in a monsters properties, you can add keys. Add a key named 'body' and give it a value. This value corresponds with a unique combination of body-group options.
*The third 'weapon' is void, you get this when a grunt dies and loses his weapon.