Object Types for Initiate Workflow are limited - How to specify Part Request?

I am creating a test workflow for a Part Request but want to use the initiate workflow option in the workflow administrator.  The problem is that Part Request is not one of the selectable types to link to the primary business object.

I've tried copy/paste, leaving type blank and searching by number. Is there a way to add additional types to the list?  As a backup, I considered if this is a dead end, just asking for the number and putting a lookup method in the workflow which will retrieve the part request and set it to the PBO. This is a generic problem since the list of objects above is short. There are more objects that you can run workflows against in the system.