You can use the raise a request on behalf of feature to open an issue on behalf of your customer. When you do that you basically are creating the issue via the portal setting the reporter to be your customer and choosing the correct request type. Once you do that GSM will send a notification to the customer that the issue was created. At this point they can reply to it to add comments to the issue and you can reply or add comments to the issue and they will get a notification.
is this what you are asking for?
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