In short, no. There is no part or easy way to lengthen your mag box.
The long answer is you can get longer OAL with some modifications.
You'll need to remove the mag box from the stock. Ditch the "L" bracket. Fabricate a retainer/floor to your centerfeed mag box. Slot the front of the mag box so that the bullets stick out the front. Then figure out a way to fix the mag box to the stock (I've used putty epoxy).