...

PopulationTables currently do not have limited support for modding, but many zone template encounters draw objects from dynamically created population tables. Check out the following tags in ObjectBlueprints.xml to see them in use: DynamicObjectsTable, ExcludeFromDynamicEncounters. Some dynamic population tables are created based on the base type of the object, so new objects will begin appearing immediately in areas generated with dynamic population tables unless they are tagged with ExcludeFromDynamicEncounters.

...

<encountertables>
  <encountertable Name="Ammo 1" Load="Merge">
    <objects>
      <object Chance="100" Number="3d6" Blueprint="My New Ammo"></object>
    </objects>
  </encountertable>
</encountertables>

...

Additional Info

Here's a thread on Steam where I discuss using encounter and population tables: http://steamcommunity.com/app/333640/discussions/3/358415738194955181/