Extend JIM to UPDATE already imported issues

We use the CSV importer to regularly import new issues from the bug tracking system of a customer.
With the wizard it was very easy to initially define a mapping between the 2 systems.

The problem is, the CSV importer of Jira can do only INSERT, no UPDATE.
So we have to manually update Jira to keep it in sync with the customers bug tracker.
Our customers do not change very often the issues after we get them, but sometimes they do
and as projects are getting bigger this is becoming too much to handle manually.

Are there any plans to add to the CSV importer the capability to update already imported issues?

Or are there any other solutions to this problem?
Are there any "standard" Jira solutions?

As I see the Jira CSV importer was created as a tool to import data from another system once,
not to keep data in sync between two systems. But with a small extension it could be made into one.
I just could not find this issue anywhere here in Jira's Jira. We need a solution quite urgently.

Regards
Bettina Zucker