Good question.

But it's sort of easy; you are telling sql server which CPUs it is allowed to use (alter server configuration set processor affinity); and you are telling the system how SQL Server is to see that collection of CPUs as split into (soft) NUMA nodes (by registry manipulation).

There's unsatisfactory user interface here: introducing registry entries with regedit or reg merge. It would be nicer to do this with a soft-numa-specific GUI and a soft-numa-specific command line, each of which could check for idiocies like using a mask which would assign CPUs from different hard numa nodes to the same soft numa node or putting a processor into more than one soft numa node. It would prevent errors arising from mistyping those long keys, and could either raise an error message if the GROUP is not specified for 2008 R2 (not needed for 2008 or 2005) or if a nonexistent group is specified, or alternatively provide a default group number (0, because most systems will have 64 or fewer CPUs).