to define custom keywords, you need to edit the policies via the Admin GUI.
You can find the details about how to do that in the AdminUserGuide.
Nevertheless, as far as I understand your question aims at the context between a source file and a workflow item.
IMHO there are two possible way, depending on the Version of Integrity you use.
in I10 or later you have the "Source Link" Field Type for Workflow items that establishes a "relationship map" between a project member and an item ID. It can be used in both directions (from source and from the workflow items). This works without keywords and is handled completely by the database.
pro: Out of the box and somehow easy to use (drag 'n' drop gesture) and the "links" are dynamic
contra: the "links" are dynamic, that means any change in your source file's member revision is reflected in the linked item. It helps to keep up to date, but it is not possible to link an item to a certain revision.
You can use the ChangePackage ID in your keywords. As change packages are the way to link change in source to items you could enable change packages for your requirement items.
I think it would be best if you discuss your option with PTC support, as it deeply depends on you local Integrity solution's implementation.