Yes, Virginia, there is a two wheel drive transfer case.
However - it is really not a transfer case in the strictest sense!
All it really does is take up the space of the 4x4 transfer case so that the rear output shaft is physically located in the same place as the 4x4 t-case so the rear driveshaft and axle are universal between the 2wd and 2wd trucks. Oh and the 2wd transfer case is geared to match the 2-hi mode of the 4wd t-case.
Here is a link to the 1990-1992 Samurai Supplementarey FSM at Ack's FAQ:
http://www.acksfaq.com/90-92Samurai_SSM.phpSection 5 describes 2wd transfer case tear-down and reassembly in detail.
Based on the contents of the document mentioned above, the ultimate answer to your question is: No, you cannot rebuild a 2wd transfer case to make it a 4wd unit. It's a WHOLE LOT Cheaper to find a 4wd t-case in a salvage yard.
BTW, your 2wd t-case (heck, the whole vehicle!) is a RARE item! It would be a terrible waste to try to convert such a rare vehicle - much rarer than tintops to be sure! I have actually seen only ONE 2wd Samurai in my travels.