No, you do not have to import the entire wallet.dat file.
You can use the console in Credits-Qt and use the command bitcoin_importprivkey to import one single private key. Issue the command and then run restart the client with -bitcoin_rescan.
Oh! I see now

Apologies for the confusion, it seemed a bit...dangerous, but after reading up some of the things posted (apologies for not doing so earlierr) the process makes a lot more sense!