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You can, and indeed many direct-drive hub systems include this, but it's not a range issue. It's more to save some brake wear. In a 'best case' scenario of many steep hills, such a system will extend range perhaps 5%. Much less in normal use.

they are sometimes, but actually that is rather pointless. The energy wasted by braking is quite negligible compared to riding at reasonable speed with some headwind. better invest in a better battery or make the bike cheaper than regenerative braking. At least from my perspective with an electric cargo bike an normal e-bike, and a super expensive speed-pedelc (45km/h) Stromer bike with regenerative braking. Because of the regenerative braking my fastest bike is actually the slowest if the battery is dead. Also it is almost impossible to go faster than the speed limit as the bike actively brakes whenever you go over 48 km/h. That's why I am selling it.

It's not worth it.

First, I believe conventional wisdom is you can recover 1-3% of total battery capacity, which in a 400Wh pack might be as little as 4Wh, or one AA battery worth.

Second, you can only actually do regen with a hub motor, but mid drive is much more popular.