Right now all our build worker images have PowerShell version 2 and 5 installed. This is default setting for Windows Server 2012 R2 and Windows Server 2016.
If you have a idea how to install another versions of PowerShell on any of those OS (at least manually), we can help you with scripting it and running it at install stage of different jobs in build matrix.
However if it requires different OS (like "Default Windows Servers" in this article), then your best bet would be running your own custom images with private build cloud. However this feature is available for Premium plan customers. We provide 50% discount for open source and trial is free.