Looking at the picture it seems like the barb is bolted into the false bottom. I'd call northern brewer to make sure, but I would imagine you could just buy yourself a 1/2" barb on a 90-degree elbow with the same size threads and replace the one they have on it. That would be my suggestion.
Your other option is to just use a 3/8" hose from the barb to the inside of your pass-through on the MLT, then install a 1/2" hookup on the outside for your march pump. If you do that, however, you're going to need to make sure you install a ball valve on the outlet side of your pump and keep it throttled back a bit. You don't want to get a stuck sparge from pulling too much vacuum on your mash, or burn out the motor on your pump.
Replacing the barb with a 1/2" ID one is still your best bet, though. And I took another look at the picture. I'm almost sure you can do that, unless they've welded/soldered the fitting in place. But like I said. Call NB and ask them about it. That will tell you what you need to do.