It does not answer your question of getting your private keys out of bitcoin-qt, but this link explains how you import a single private key into MultiBit:
If you have more than one private key it is simply more rows in the import file.
The date alongside each key is a bit non-obvious. It has to be before the date of the first transaction using that key so that the blocks with the transactions in can be found. I recommend just finding the first transaction in your wallet and putting the date one day before it in on every row. (Using a day earlier avoids any time zone problems).
Note that the import only knows about the sipa format private keys (the ones beginning with a 5). It does not understand compressed keys. ( What is a compressed Bitcoin key? )