To do that you're gonna need a tranny, center diff, driveshaft, rear diff, rear halfshafts, new rear susp, which is probably over the price of what you paid for the car or what it would cost to get a quattro model.
If you really want to keep that car, and really want AWD in it, im sure it could be done with the right parts, funding, and expertise. However, if you don't have extra cash floating around, lots of tools, and a good mechanic(or are one yourself) I would highly advise saying **** it and getting a different car.
Good luck my fellow 12v owner, I was in almost the same boat, having gotten an auto 96 A4Quattro through family convenience and wishing for a manual that I could mod. I was thinking about a tranny/motor swap but soon opted for buying a 2000 S4 6speed and keeping the A4 as a dd, your fwd will get better mpg than its quattro counterpart on the bright side, and is a bit quicker in the top end due to less rotational mass in your drivetrain.

 

take your car to a used car lot that has an AWD 1.8TM and just simply trade it in...it will save you thousands....and the way gas prices are lookin....a 1.8T will pay itself off in a couple years....well, gl!

 

Back half of the FWD and quattro cars are completely different, FWD uses the rear suspension from a passat while the quattro has the suspension located on the inside of the wheel well. To convert the FWD car to quattro you would have to cut the back half of the car off and replace it with a back half from a quattro A4/S4. This is not as simple as one would think. Just better off selling the car and buying a quattro.