Another possibility is to download Adobe's free DNG converter. Use it to convert the D600 files to DNG, then open the DNG files in PSCS5. It's another step and generates another file, but DNG is a raw format that can be read by ACR and any other raw conversion software.
Since DNG is still raw, you'll keep more flexibility in CS5 than you'd have with a TIFF.
On the other hand, the advantages of ACR in CS6 are major, letting you do much basic editing non-destructively directly on the raw file than was possible in CS5.
Lightroom 4 also follows the same non-destructive approach. Though it's less powerful than PSCS6, it offers a great amount of file processing power--as much as many files need--all of it non-destructive.