Larry,
Much depends on how mechanically inclined you are, and what tools (and money) you have available. To build something like the "monster smart" that is floating around the internet, you need to engineer a carriage to lay the smart on. That carriage is what you actually mount the axles and drive-train to.
If you are looking to just mod a smart to 4x4 (no real extreme lift, etc.) you will still need to do basically the same thing.
You need to locate or fabricate the appropriate axle shafts, motor mounts, etc. etc., which can cost a fair amount of money, and if you do it yourself, time as well. You need access to a decent machine shop (lathes, mills and a good welder, for a start).
I'm sure if you do search the internet, you will find plans, or at least a guide, on how to build a 4x4.
But, should you opt to do this, PLEASE take lots of pics, and post them here. We would all love to see it!
Thanks