You do realize that continuously taking power from the forward motion of the bike to run the lights is going to slow it down, and energy regained that way is wasting more than you get back, because of all the conversions from battery to controller to motor to controller to battery, unless you were trying to stop.
So running your lights off of forward motion will just drain your batteyr faster than just directly running the lights off the battery (even via a DC-DC).
There's a lot of threads on this kind of thing.
That said: No, there are no separate systems available to do just braking / regen, that you could hook up to a motor that's already being used by a controller as a motor.
You can design and build something if you like, and there are threads discussing the problems of switching back and forth between traction controller and braking controller, but it's WAY WAY less complicated to just get a cotnroller that does regen in the first place (especially since the easiest way to get a separate regen braking system is to just use a regen-capable controller...which if you already have it, you should just use it as the only controller).
If you want you can use a separate motor as a generator to run the lights, but it's simpler to just by a dynamo hub wheel. Of course, either of these solutions is just another version of the problem noted at the start of this post--you're just wasting power out of your battery. It probably wastes less power to just use giant resistors to drop the battery voltage down to whatever the lights run on, and directly run them off the battery (rather than using a DC-DC).