FYI: the global inbox is located at /Users/*yourusername*/Library/Application Support/DEVONthink 3/Inbox.dtBase2. The inboxes associated with individual databases are included in the respective database files.
A simple way to do what you want to achieve is to copy the file referenced above to an external drive and to put it back after the fact. If you want to keep other bits of DEVONthink (scripts, settings…) you may want to see this post and this post.