If this is possible would this produce the key to the genesis block? I am very curious why you'd see bitcoin spinoffs like BCH have different wallet formats than bitcoin.
As part of the source code for bitcoin, the genesis block reward is unspendable so you wouldn't be able to spend it unless you changed that and you might have to fix any security loopholes it could produce.
Although if you meant just making your own genesis block, you can of course do that but it's normally hard coded into clients that are distributed - potentially either for security reasons or to reduce errors/error handling as soon as the client is started.