Are desktop wallets transferable? I know you can make a transaction across the blockchain to move your Bitcoins from say Bitcoin-QT to Multibit, but is it possible to import my wallet.dat file from Bitcoin-QT to another desktop wallet without preforming a transaction?
You can simply copy .bitcoin/wallet.dat to another place, and use it like HOME dir for bitcoind/bitcoin-qt. 

It is like a copy the whole bitcoind data. 

Other files in .bitcoin/ directory are just a logs and configs with database, which will be downloaded from network automatically by your new client program.