I'm not sure which ports the DS uses for its internet connection, but, if you find them, you could simply port forward them to your pc with the USB adapter.
Honestly though using WEP instead of WPA security settings for a home network wouldn't make a bit of difference. No one is going to sit outside your house with a laptop and take a few days to hack into your system. It just doesn't happen.
For say a government agency, like where I work, yeah, OK, I can see that. For a home network, its a little over kill.