You can add custom data default values in the cat or prj. This will fall through to the items.
Then the Spec View palette has the Custom Data tab, which you can then change the custom data default values again if need be.

If you need to change the custom data values after the items are placed, the fastest way would be using MTOWorx, where you can modify multiple items at once.

As far as mapping data from CWX items to a title block attribute, it is possible. There should be a post on the forums on how to link the two together. But it's much easier to use ISOWorx.

We simply look for the attribute tag from the title block and match it to data in the Excel, database, or cloud-based line list.