Brining a piece of brisket for corned beef.....Can I freeze it before cooking?

Yes, you most certainly can freeze it. But you need to remove it from the brine first, and freeze it without that. If you can vacuum seal it, all the better- but if not, try to get as much air out as possible. When it comes time to cook it, thaw it in the frig and proceed with your favorite method, using a bit of fresh pickling spice. The experts say it shouldn't be frozen more than a month, because texture will suffer. But I've had it frozen for just at two months, and it was just fine.