As a workaround as the Update Manager cmdlets do not support baseline groups,
I create my own baseline group in the PowerCLI script:
# Get MyTestCluster
$cluster = Get-Cluster MyTestCluster
# Create a Baseline Group from multiple baselines
$baselines = get-baseline | `
Where {$_.name.contains("ESXi5 Upgrade") `
-or $_.name.contains("- Dell -") `
-or $_.name.contains("VMware ESXi 5.x Patches")}
# Start remediating the cluster
$Cluster | Remediate-Inventory -Baseline $baselines `
-ClusterDisableHighAvailability:$true `
-ClusterEnableParallelRemediation:$true `
-HostIgnoreThirdPartyDrivers:$true -Confirm:$False
Based on the baseline names, I get one upgrade baseline (ESXi4.1 -> ESXi5) and a baseline with the latest patches,
and a vendor specific extension baseline. Update Manager puts these automatically in the correct order and upgrades and patches all hosts