To address the question of adding Linux real memory dynamically: The z/Linux support for dynamic addition of real memory has been provided to the open source community fairly recently. (I don't know the exact date.) When that support becomes generally available is up to the open source community and the commercial distributors of Linux.
There is no support for dynamically adding real memory to z/VM itself or dynamically increasing the "guest real" memory (that is, "DEF STOR" memory) for guest of z/VM in any announced release of z/VM. - Bill Holder z/VM Development, IBM
