Generator connected direct to an Unused Breaker
Yes, but not safely nor legally. There are plenty of resources available explaining the risks of doing so and the right way do it. Here's one:

This is dangerous.
It is an electrical code violation.
It is illegal in most places.
It is a fire hazard. The power created by your generator is generally greater than the rating for the receptacle, wire and breaker [when connected using a male-male extension cord].
If you don't disconnect your main breaker it can shock the power company linemen - and you will get sued.
You can easily get shocked because the male plug prongs are exposed [when connected using a male-male extension cord].

I realize that the statements above refer specifically to the male-male extension cord to an outlet approach, but the legality and risk to linemen points are valid.

Plus if the insurance company determines that you were negligent or
reckless in using such a generator connection, they can refuse to pay
out on any insurance claims that result. The best thing to do is to
use an approved transfer switch or lockout device. ~Johnny