Adding Closed Issue from Closed Sprint to Epic

Customer would like to request for a feature that enable them to add Closed issue from closed sprint to the Epic. Refer below to the actual quote from customer:

Our intention is that we want to have complete epic burndown charts (also for the past).
Problem was that epics and epic burndowns were first available since on one of the latest greenhopper releases. So our epic burndown chart is not complete because some issues were not assigned to an epic.

Therefore we need to add closed issues of closed sprints to an epic to have the burndown chart complete.

As according to the documentation, adding issues to Epic requires the issues at least to be visible on the planning board.

Would like to suggest to enable a field or something similar to bulk edit option to include the filter result into an epic.