Normally, importing mailboxes (i.e. .dbx files) works well in Outlook Express 6, as long as the .dbx file is writable(!). Sometimes though, you get this message:

No messages can be found in this folder or another application
is running that has the required files open.
Please select another folder or try closing applications that may
have files open.

If that happens, I've found the following rather convoluted recipe works. For the purposes of this explanation, I'm assuming that you want to import an old copy of Inbox from a backup.  For other mailbox names, adjust the instructions to suit.

1. Copy the backup copy Inbox.dbx from wherever it's stored to somewhere convenient — I'll assume you put it on the desktop.
2. Rename the .dbx file to something that doesn't already exist, such as Inbox2.dbx.  This is important — if you leave the file as Inbox.dbx, you will overwrite and lose your current Inbox.
3. Open Outlook Express.
4. Create a new folder with the same name as the file on the desktop (but without the .dbx extension), i.e. Inbox2 in this example.  This folder should be within 'Local Folders', and not a subfolder of any other folder.
5. Force Outlook Express to create the mailbox for the new folder by moving an unwanted email (there are usually lots of those lying around) into the Inbox2 folder.
6. Find out where Outlook Express is storing mailboxes by going to Tools / Options / Maintenance and clicking on the Store Folder button.
7. Close Outlook Express.
8. Open the folder containing Outlook Express's mailboxes that you found in step 6.  Normally, it's in C:WindowsDocuments and Settings<your user name>Local SettingsApplication DataIdentities{<lots of random letters and numbers}>MicrosoftOutlook Express.  If the Local Settings folder isn't apparent, you'll need to change your Folder Options so that Show hidden files and folders is selected.
9. Drag the file Inbox2.dbx file from the desktop into the mailbox folder (where all the other .dbx files are, including the new Inbox2.dbx).  Windows will ask you to confirm that you want to overwrite the existing Inbox2.dbx file: say Yes.
10. Open Outlook Express again, and go to the Inbox2 folder.  It should contain all your old emails.