It doesn't work that way -- the additional load you put on the existing motor would be more than any energy created.

What you're trying to create is basically a perpetual motion machine -- and it just doesn't work.

The only time what you're talking about makes sense is when you're intending to slow down already, and many electric bikes already have a feature that recovers some of that energy -- it's called regenerative braking. It's also found in hybrid cars (and really, it's the #1 reason for making them hybrid in the first place.)

In any event, if you're after regenerative braking, you don't want to use fans or turbines -- you want to use a generator coupled to a wheel. And since electric bikes already have one -- the main motor -- they just use that. (Motors and generators are basically the same thing.)