You can easily import MBOX files to Yahoo! using MacUncle MBOX to Yahoo Migrator. The tool supports Mac OS Machines.
Start MacUncle MBOX to Yahoo Converter on your Mac Computer.
Select MBOX file using Add File(s) or Add Folder(s).
Choose the desired files from the software panel.
Click on the Yahoo in Select Saving Option and then enter Yahoo Email address and Password and IMAP Port and IMAP Host.
Then, press Export button to migrate MBOX to Yahoo account.