Yes you could. It would be tough to get it to work on a Raspberry Pi and you would need external circuitry to increase the frequency from zero to VHF (or wherever you wanted) and also to filter the signal so that it was clean.
It almost certainly would also be illegal. Radio broadcasters have to be licensed and there is no publicly available license for entertainment stations. Even the license-free bands cannot be used for entertainment and nobody has a receiver for those bands anyway.
If the power is low enough, then it might be legal. Check your local laws. But then why would you want multiple stations?
If you mean can you make an Internet "radio" station that sends out more than one channel at once, then yes, and it would be much easier.