Assuming the controller supports single-input braking* then you could use a cable from your brake handle to one of those cable-operated potentiometer throttle units, which can be used to create whatever voltage range your controller requires for the regen braking. (including reversed voltage, starting high and going low, simply by reversing which outer leg of the pot is connected to + and which to - .)
If your brake handles are already used for cables, you could add a second brake handle for this, either with a custom mount to put it exactly parallel above or below the existing one, or just inboard or outboard of it. (like I presetnly have on SB Cruiser's left handlebar).
Alternately, you can use a lever throttle that is "under" the handlebars, it's like a thumb throttle (and usualy thumb operated) but it doesn't rotate circumferentially around the bar tube, it pivots left/rigth under the bars.
Or just use a thumb throttle, but it's harder to make room for on hte bars; forces moving other controls inboard, or have to put it inboard and use a longer thumb tab to get "around" the other controls without moving your hand on the grips.
* meaning it doesn't require a switch to turn on braking and then a separate voltage input, usually the actual throttle, to control the amount of regen