You can try the following:
Set up with all your wireless deployment ( MXw and the rest of the Meraki Estate ) with:
Exact same SSID / Exact same password
With this you can achieve some kind of " manual roaming effect", but its not guarantee your end-points will be disconnected when they shifting from one AP to another or the MXw.
The MXw is only a "one-stop solution" , not designed for client roaming.