Have a Nighthawk X4 and trying to add Nighthawk Mesh. Possible?

I have a existing AC2400, and was told by Best buy I could mesh the new Nighhawk mesh system with it so I can roam and keep one SSID. I just bought the two pack and was under the assumption that it was two mesh extenders that connected to the X4 but appears to be a router and one mesh AP. It looks like I can set the Mesh router to AP mode and possibly connect to the existing X4, but I am pretty sure the roaming wont work properly. Also best buy only had the two pack and now looking around Amazon has a three pack with two mesh AP's which is what I was trying to do(one upstairs, one in back of house to extend outside for cameras and streaming by pool.) 

It does not appear that they sell another mesh AP, so I am wondering if I should just get the 3 pk off Amazon and return the 2 pk to best buy. Has anyone gotten the mesh router and AP to play nice with an existing X4 Nighthawk?