Re-buy old songs for keyboard/vocals.
If you want keyboard and vocal harmony parts added to your old Rock Band songs then you’re going to have to buy them again.
Specifically, you’ll need the Rock Band 3 version of the song charged at, presumably, the default DLC price – the Harmonix website doesn’t clarify.
The only upgrade available for old Rock Band songs is to add the Pro Guitar and Bass tracks, a procedure that costs 80 Microsoft Points/£0.59/100 Wii points.