If it's a bicycle dynamo then it most likely isn't a dynamo but an alternator.
The peak voltage is 8.5V so you might be able to charge a 9V battery if you work really hard and use a Schottky rectifier especially if it's really a 7.2V or 8.4V battery.
The battery specs is 9v but the current doesn't have to be specific. It must just be sufficient to power a 9v radio. Is it possible to step-up the 6.2v to 9v?
the dynamo should just charge the battery. I'm thinking of using a 3W 6V dynamo. is there a schematic I can use to see how to connect everything together?
The circuit will be much more complicated, if the voltage is 12VDC.
I think you should make sure you know whether it's AC or DC first.
If you're not sure, connect two LEDs in reverse parallel with a 470R resistor in series and connect it to the output of the generator. If it's DC only one LED will glow, if it's AC they'll both glow.