You simply can't: the coinbase tx of the genesis block is not spendable. You have to craft a blockreward function in a way that all your premine is created in the block #1. Then you have to find the solution for this block (the same way as you did for genesis block)
Yes you can configure your blockchain to mine 100% of total coin in first block and as nobody will be there mining your coin, you will get all of the coins in your wallet.