In reality there is nothing difficult or hidden in creating a genesis block, it just need custom software be able to generate the genesis block document. But you can also create it by hand.
You (and the initial users of your currency) need to create Identity, Membership and Certification document like you’ll do it on G1, but with your currency name and without publishing it (since there is no chain to publish to). Then you create your genesis block document with all those and the currency parameters. Then a first node can publish this genesis block, and the users sync to it and it should work.
All these steps can be inferred from the Duniter protocol documentation which explain the validity rules of each documents. If all your documents are valid, then it should work.
I say it should work since I’ve not tested it, but by reading the protocol specifications it should be correct. If it’s not (and the G1 genesis block has not been generated like this), let me know and I’ll be happy to read the explanation and to donate some G1.