Also to convert 12 to 36v with a buck converter (with an isolated output) is gonna cost you at least $300
(all the eBay ones you might see for $15 are non-isolated.)

So alternator charging a battery at 300w (ain't gonna happen with human power on a bicycle crank) would produce 21a, then inverter at 90% efficiency would produce 270w or 36v 7.5a. That would definitely power the hub motor, however your probably using 400w of human power (IF it was possible to pedal at 400w and turn an alternator fast enough to generate 300w out of it - which really ISN'T possible, but IF it were) and get speeds of let's say 20mph out of your hub motor..
OR
Just pedal a bicycle with gears at 400w and the bicycle moves at 20mph with human power.