It is possible to make a series-wound motor brake - not generate - but you need to set up wiring to do that (and it's generally not a good plan).
Every electric forklift does this. It's called "plug braking". Even my 1969 York forklift with SCR and capacitor snuffing speed controller has automated plug braking.
More or less, you slam the motor into reverse. The motor doesn't really care which way it's spinning, it just cares how you're trying to accelerate it. If you're going forward and you put it in reverse, it'll decelerate down to a dead stop and then start reversing. I think there was some handy circuit to have a reasonable amount of braking (so you're not actually putting it into reverse, the controller understood the concept of braking) by default.
In this sense, not only is the braking not regenerative, you're actually spending new energy to counter the old energy. The result is the motor getting, I dunno, presumably twice the heat dump on it as plain acceleration.
You can also do resistive braking, where you use the motor windings as a resistor to slow down, but without adding new power into them. I don't think that's technically called "plug" braking at that point. You'd never come to a complete stop, but it would bleed some energy off as heat. Less effective but more efficient than plug braking because at least you're not spending new energy on it.
All in all, okay, yeah, probably not a good plan when your car is going to have brakes anyways. Brake pads are cheap. If you're not getting any extra energy out of the braking, I'd say spend your brake pads, don't risk your motor and controller smoking. It's all going to end up as heat anyway, might as well put it into a system that has a 100 years of engineering behind being safe and effective at turning motion into heat.
