The kind folks at the MTV Multiplayer Blog gave its readers a series of steps to follow in order to play the Rock Band 1 songs through Rock Band 2 when it's released.
Here's what they had to say:
Yes, this also means that anybody who rents the first game and purchases the second game can export those songs to their hard drive as well. It should be known that due to licensing issues, it's possible that not every song from Rock Band 1 will make it to your hard drive, but most will.
These steps were only confirmed for the Xbox 360 version of the game (releasing September 14th), but will probably also apply to the PS3 version given its similar nature.
Source
Here's what they had to say:
• The day that Rock Band 2 is released, the first title will receive a small update. This update will add an "Export" feature to the game's main menu.
• Selecting this option will bring up a prompt for a password (provided to you in each copy of Rock Band 2).
• You will then have to pay $5, at most, to transfer those songs on to your hard drive.
• After the transaction is complete, the songs will migrate to your hard drive, and can then be played in Rock Band 2.
All songs that were downloaded as DLC won't have to undergo this process as they're already on your hard drive!• Selecting this option will bring up a prompt for a password (provided to you in each copy of Rock Band 2).
• You will then have to pay $5, at most, to transfer those songs on to your hard drive.
• After the transaction is complete, the songs will migrate to your hard drive, and can then be played in Rock Band 2.
Yes, this also means that anybody who rents the first game and purchases the second game can export those songs to their hard drive as well. It should be known that due to licensing issues, it's possible that not every song from Rock Band 1 will make it to your hard drive, but most will.
These steps were only confirmed for the Xbox 360 version of the game (releasing September 14th), but will probably also apply to the PS3 version given its similar nature.
Source