Change data capture - how to know who made the change?
I filed a bug about this, but it was closed as "by design."

http://connect.microsoft.com/SQLServer/feedback/details/283707/cdc-options-to-capture-more-data-username-date-time-etc

Unfortunately you will have to use another technique (e.g. SQL Audit or a trigger) to get this information (and some custom code to try to correlate it to the CDC data as much as you can). I wrote about this deficiency also in my chapter "SQL Server Audit, Change Tracking, and Change Data Capture" in the book SQL Server MVP Deep Dives (volume 1).

I'm sorry that I don't have a better workaround for you but the built-in capabilities of CDC just aren't going to meet your requirements. :-(