MBox Office is a simple mbox viewer. It lets you restore your archived .mbox archives to any IMAP-supported email provider. You should be able to open mbox files using File -> Open.
Features
Tested with iCloud, Gmail, Google Apps, Microsoft Exchange--it should work on most email accounts
Supports multiple email accounts
Native email browsing interface where you can restore specific emails
Batch email restore--select all emails and click on "Restore Selected Emails"
You import individual emails in .eml format to your existing .mbox file. You can use all the above mentioned features with it.