I've just committed the changes to add nwkey keyword to ifconfig(8).
The available drivers are wi (WaveLAN and PRISM-II), and awi (AMD/PRISM-I),
for now.  (ray driver looks no support to wep).

Here is the usage from man page:

     nwkey key       (IEEE 802.11 devices only) Enable WEP encryption for IEEE
                     802.11-based wireless network interfaces with the key.
                     The key can either be a string, a series of hexadecimal
                     digits, or a set of keys in the form n:k1,k2,k3,k4, where
                     n specifies which of keys will be used for all transmit-
                     ted packets, and four keys, k1 through k4, are configured
                     as WEP keys.  Note that the order must be match within
                     same network if multiple keys are used.

     -nwkey          (IEEE 802.11 devices only) Disable WEP encryption for
                     IEEE 802.11-based wireless network interfaces.

Enjoy!

Atsushi Onoe