There is a special tool that should be used to remove the manual addition valve. I've seen it at my local dive shop (
Fill Express -- Nitrox & Technical Diving Specialist Serving Greater Fort Lauderdale -
Dive Rite Express), but I don't think it is an item they had for sale.
One note: I would imagine that Dive Rite has intentionally placed a different type of valve on the diluent side, push vs. slide, to help ensure that divers did not accidentally hit the wrong button.
Consequently, I would seriously consider whether or not you want both valves to be the same, because I wouldn't want them to be the same.