I've seen a few threads on the forums regarding enabling the DLC weapons outside of the DLC missions, and I'm not sure anyone has successfully figured out how to do it.
As has been pointed out elsewhere, the DLC weapon resources are loaded into mission memory with the DLC missions because there was not enough preload memory to permanently load them with all the standard weapons in the game. The main challenge will be getting the DLC weapon resources bundled up and loaded into the item preload mempool or some other permanent mempool. I know Mike (Knobby) is working on some changes for a new SR3 PC patch that will remove many of the limitations restricting modders adding new content. I will have to ask him specifically about whether the item preload mempool will be increased as part of the patch.