?
This would be very easy to do with a PowerShell script, so as an immediate solution, I'd suggest to do that. It'd also be very easy to patch/modify that particular action (CreateIisAppPoolAction.cs)
That being said, this all applies to the "v4 execution engine", which will very soon be replaced with an indescribably better execution engine (the one that Otter currently uses). When we migrate the "v4 App Pool Action" to the "v5 App Pool Operation", it will basically look like the Ensure-AppPool Operation.
So, as you continue to explore BuildMaster, do keep this in mind :)
If you're doing server/infrastructure configuration (which, creating applications is definitely a part of), I'd strongly recommend looking at Otter as well!