Can you make a song on Rock Band?

Yes, You can. You can obtain information from Rockband.com. Tools used are Reaper, Magma (Rock Band Link to Xbox 360 tool) and a subscription $99.00 per year to have the ability to publish to the rock band creators network. One thing, even if you are doing it for fun, it seems like you still need to join the creator's network to "enable" the Magma linker tool and allow copying songs to your Xbox 360 to demo in audition mode.