Use the github api for issues http://developer.github.com/v3/issues/#list-issues-for-a-repository to import tickets into allura. You can use ForgeImporter's GoogleCodeTrackerImporter (at time of writing, cj/6464 on the apache repo has the latest code for it) and TracTicketImporter as reference.
Import all the fields & values possible, including comments, events (e.g. status changes) and attachments. See the Google Code and Trac importers for examples of setting up the custom fields as needed, and referencing the remote users (keep the tickets & comments as anonymous and add identification with link into the body).
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