Open CES and create a new rule set under Schemes. Open the new scheme and under Via Assignments you will see a block containing a "..." button and (default). Click the "..." button and pick a new via. In your board layout place a Rule Area shape around your BGA and attach the new scheme to it. When you are routing inside that Rule Area shape, you should get the new via. You may also want to set different clearence rules for traces, pads and vias that are within that shape.