That's just an array of your VM morefs for the affinity rule (or disaffine rule).
You should be able to set your operation to "add" and then just provide an array of vm morefs that would be appended to your existing rule.
$groupSpec = new ClusterGroupSpec();
$groupSpec->{operation} = new ArrayUpdateOperation("add");
$groupSpec->{info} = new ClusterVmGroup(name => $drsVmGroupName);
$groupSpec->{info}->{vm} = [$vmMoref];
$spec = new ClusterConfigSpecEx();
$spec->{groupSpec} = [$groupSpec];
$cluster->ReconfigureComputeResource_Task(spec => $spec, modify => 1);
Check my logic on the add operation there, typed this up from memory and a few glances at the API docs. You could also add more than one VM if you wanted to here as well (which is likely what you want to do for an affine rule).