[DCP-o-matic] DCP re-packing

Hi all. Is it possible to "re-pack" an existing DCP without re-encoding the video? I have a couple of DCPs which have problems with the sound so I'd like to make a new DCP using the existing DCP video MXF and replace just the audio tracks. I think this came up before and I thought it was supported in DCPomatic 2. But I tried it in v2.7.1 and couldn't work out how to do it. Importing the J2K MXF file works fine, but with colour conversion set to "none", when it came to make the DCP, from the slowness of the process it appeared that the video was being re-encoded rather than copied. Did I use the wrong options, or is this not supported in DCP-o-matic at present? Any help much appreciated... Jim