How to set up custom arguments for the "queue build" screen with drop down choices?

Question

Basically I've been following Jim Lamb's article here: http://blogs.msdn.com/b/jimlamb/archive/2009/11/18/how-to-create-a-custom-workflow-activity-for-tfs-build-2010.aspx?wa=wsignin1.0

Near the bottom he discuses how to add arguments on the Parameters tab on the Queue Build. I have an option that could benefit from a list of choices rather then typing in straight strings, much like how "Logging Verbosity" already works on that screen. Is this possible?