Currently not with the cmdlets afaik, but with the API everything is 'posh'ible.
This will create a dummy VM and VMHost group, and then create a rule with both groups.
You can adapt the groups (for example add ESXi nodes) or change the type of rule
$cluster=Get-Cluster-Name $clusterName
$spec=New-Object VMware.Vim.ClusterConfigSpecEx
$newVMGroup=New-Object VMware.Vim.ClusterVmGroup
$newVMGroup.Name='DummyVM'
$newVMGroup.UserCreated=$True
$newVMGroup.VM=$null
$groupSpec=New-Object VMware.Vim.ClusterGroupSpec
$groupSpec.Operation=[VMware.Vim.ArrayUpdateOperation]::Add
$groupSpec.Info=$newVMGroup
$spec.GroupSpec+=$groupSpec
$newVMHostGroup=New-Object VMware.Vim.ClusterHostGroup
$newVMHostGroup.Name='DummyVMHost'
$newVMHostGroup.UserCreated=$True
$newVMHostGroup.Host=$null
$groupSpec=New-Object VMware.Vim.ClusterGroupSpec
$groupSpec.Operation=[VMware.Vim.ArrayUpdateOperation]::Add
$groupSpec.Info=$newVMHostGroup
$spec.GroupSpec+=$groupSpec
$newRule=New-Object VMware.Vim.ClusterVmHostRuleInfo
$newRule.Name='Dummy Rule'
$newRule.Mandatory=$true
$newRule.Enabled=$false
$newRule.UserCreated=$True
$newRule.VmGroupName='DummyVM'
$newRule.AffineHostGroupName='DummyVMHost'
$ruleSpec=New-Object VMware.Vim.ClusterRuleSpec
$ruleSpec.Info=$newRule
$spec.RulesSpec+=$ruleSpec
$cluster.ExtensionData.ReconfigureComputeResource($spec,$true)