If you have a MultiBit private keys export file you can reimport them into a MultiBit wallet.
To import the private keys into a wallet, do the following:
In the 'Wallets' side panel choose the wallet you want to import the private keys into.
Select the menu option 'Tools | Import Private Keys'. The 'Import Private Keys' screen appears.
Choose the private key file to import by clicking on the 'Choose import file' button. A file chooser opens and you can select the file you want to import. You can also import blockchain.info wallet backup files by changing the file type in the 'File format' drop down combobox.
If the import file is not password protected, the file is read and the number of keys and replay date for the file are shown. If the file is password protected, enter the password in the 'Password' field and press 'Unlock' to see the same information. If you have selected a double encrypted blockchain.info wallet backup a second password field will appear. Enter your second password and press 'Unlock' once more.
To import the private keys to the wallet specified, click on the button 'Import private keys'. When the import of the private key file is successful a message appears.
If the import of the private key file fails for any reason the error message is given on screen.
After the import, MultiBit then replays the block chain from the replay date to find the transactions for the new private keys. For blockchain.info wallets the block chain must be replayed from the first block. This can take up to two hours on a wifi connection (depending on the speed of the computer you connect to). It is best just to leave MultiBit to sync the block chain on its own.
This is not supported in v0.4.x. The easiest way to achieve it is to simply perform a standard Bitcoin transaction to the address provided by MultiBit.