How to Create a CSR in Powershell/iControl using an Existing Key

So I answered my own question by re-reading the description of the function more clearly ...

From: https://devcentral.f5.com/wiki/iControl.Management__KeyCertificate__certificate_request_generate.ashx

“Generates the specified certificate signing requests. This assumes that each of the associated keys, having the same identification as each certificate request, has already been created.”

So it turns out you don't need to create a new key at all, as you don't specifically need to reference it when creating the CSR, you just need to create the CSR with the same name as an existing key.