Hi Jason.
Unfortunately I don't believe we have this capability.
Import and Export Data | CA Agile Central Help
In that page, see where it says: "Existing work items are not overwritten, but instead are treated as new work items".
The imports we provide both from Agile Central as well as from the Excel Add-In do create new items, they're not really updating existing ones.
If anyone knows of a way - please reply and let us know.
My suggestion though is that you post is as an idea to a future feature request. You post it on our ideas site at: https://ideas.rallydev.com (if you have no username for that site then request one via email at: ac-ideas@ca.com ).
Thanks,
Sagi