You just need to cut and paste the text and use the [ quote ] [ /quote ] tags. The opening tag takes link, topic, and date parameters.
Yeah, that's the hack I have to do sometimes. But it's really annoying if you use a lot of markup. For example, I would have to re-implement things like lists, links, and emphasis. It would be really convenient if I could just get the original markup of any post regardless of whether it is in a locked thread or not.