For anyone else struggling to find a good solution, the free ebook Take Control of DevonThink has a good solution. It says:
If you use Microsoft Outlook for email, you can also send messages or mailboxes to DEVONthink without leaving your email client. In this case DEVONthink relies on AppleScripts to do its thing.
The scripts are found on the system-wide Script menu on the right side of your menu bar. If you don’t see it there, open the Script Editor utility (found in Applications/Utilities/), choose Script Editor > Preferences, and select “Show Script menu in menu bar.” (You can then quit Script Editor.)
When you’re running Outlook, two DEVONthink-specific scripts appear at the bottom of the Script menu, both of which are self-ex- planatory:
• Add Mailbox to DEVONthink
• Add Message(s) to DEVONthink
Select the mailbox(es) or message(s) you want to import, and then choose the appropriate script from the menu.
Worked like a charm.