Hello, and welcome to the user-to-user forums here at Hak5.
First off, two big disclaimers:
1. Obviously, you're going to void your warranty if you try to modify your device yourself.
2. I haven't yet ordered a LAN Turtle - that is next on my list of hak5 gear. TBH, it's also probably the last item on my list, as I have already bought the other items I want. As such, my info presented below is my supposing, and not in any way definite or concrete.
That said, here we go! To the best of my knowledge, the original LAN Turtle, LAN Turtle 3G and LAN Turtle SD all use the same main board. From an engineering standpoint, this just makes sense. I would fully expect the 3G add-on board and the SD add-on board to be interchangeable. Just designing one main board, with a common expansion connector (or solder points) just makes things easier all-around.
I am aware that Hak5 replaced the earlier design that had a integrated USB Male port, with one that features a pig tail. I don't have sufficient info, but I don't imagine that this is related to the transition from the original model to the SD model. The USB connector change could have been (and I expect it was) to fix an engineering oversight - to take the extra weight/torque off of the computer's USB port. Although the early models probably aren't terribly heavy, they're long and bulky enough that it wouldn't take much more to snap off the USB connector.
At this point, you're probably saying "thats all great, but it doesn't answer my question". As for that, my apologies. I tend to get a bit carried away. If it were me, I would contact hak5 support and verify first if it can be done with the model you have, and second if they could just sell you the add-on board. The devs at hak5 know their own product better than anyone else - they designed them! Also, if you obtain and correctly attach their pre-designed board, you won't need to worry about device drivers, as they'll already be onboard in the stock firmware.
I hope that some of this helps.
P.S.: For what its worth, it should be fairly trivial to add your own pig-tail on the USB port - it sounds like you know what you're doing. Just desolder the existing USB, and add the cable in the length you want.