Might have been asking for trouble, but I just back fed the house panel through a spare 50A breaker. When on grid power that 50A was shut off. If the power went out, shut off the house main, turn on the 50A going to the genset and start the genset. When grid power came back on, reverse the process. Genset off, 50A breaker off, house main back on. I only had to do this a couple times, but it worked fine.
Yes that does work fine until you get the order wrong and backfeed the power company's transformer and the lineman thought they were working on a dead circuit suddenly have 2,200 volts or more.