Adding new read-only attribute to work items

When a work item (e.g. a defect) is resolved it has a read-only date for that resolution. I want to add another date attribute that will provide a read-only date for other state transitions.

For instance, when a defect is resolved by development there is a resolved date (which is read-only). When that defect is validated by test I would like to add a new attribute, which would be read-only and represent the date stamp for when the defect was "resolved by test".

In looking through the documentation it would appear that I will need to create a custom Eclipse plugin for this. Is this true? How would that equate when an item is transitioned via the web? Or am I misunderstanding?

Can someone suggest the easiest way this could be achieved...