I need to manually edit some of the files inside of the .cmp file created by stsadm export command (or SharePoint Designer backup). I can extract the files and make the edits, but don't know how to properly re-compress the files so that the resulting .cmp file is valid for use with stsadm import.
The format for CMP files is Microsoft CAB. So if you can find an archiving utility that will recompress to that format it may work.
Be careful as what you're doing probably isn't supported. Make sure it's thoroughly tested!