I think this question is actually 2 questions
1) can I purchase xenon lights from the dealer? Yup...but it's gonna cost you. I suggest you contact Ry because he sells a name brand xenon kit with high and low beam functions at a low low price (I'm gonna order from him on Saturday when I take delivery of a 95 Altima se 5sp).
2) if you purchase a xenon kit (like from Ry) can you install it yourself? I'm pretty sure you can. It should consist of wiring a box to the battery and the headlight trigger wires, then throwing in the xenon bulbs in the special holders.
If you get the dealer kit, it'll probably be even easier: plug and play a new headlight assembly.
send Ry a private message and ask him about his high/low HID kit. Then call GregV @ Mossy Nissan cuz he gives discount to Altimas.Net members.
Good luck and let us know how it goes!
Jae