Unfortunately this functionality is not currently implemented in Crucible, but you have other options.
By clicking in the Prefs dropdown while diffing a file, you can choose to list the source comments in three different ways:
I would recommend creating a new issue in our development issue tracker for this functionality to ensure our development team is aware of your request:
For more information on how Atlassian implements new features and improvements please see the following document:
My apologies that I don't have a solution for you at this time.