CSV imports in Jira can be used to both add new issues as well as update existing issues. If your CSV file has set the issue key field and you map this during the import, then yeah, it could be done.
One potential problem I could see there when doing this is that you could inadvertently overwrite other data that exists on that CG-387 existing issue in Jira. It all depends on what fields you map to during the CSV import process. If you only map fields 1, 2, 3, then only those fields get imported/updated on existing issues. But let's say you also map a dozen other fields and the CSV file you have does not have any values for those fields. Well the import process in Jira will still try to write data to those fields if you map them during your import. This can be a problem because a number of fields in Jira are required fields and hence you need to define them when creating new issues. I am thinking of Summary for one.
What I would recommend would be to first find your existing issues in Jira that you want to possibly update via CSV, and then export them via the issue navigator in Jira. This way you have that CSV data for those issues. You could then merge this CSV file with your other CSV data that you want to use to create new issues. Easier said than done in most cases, I know. The hard part here will be to make sure that you get all the fields arranged correctly between these two CSV data sets. Once you do that, you can then do a single CSV import that could both import new issues and update existing ones.
However I would recommend creating a backup of your Jira data before you try this.
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