Use the sub-generator generator to create a sub-generator. Add the name to the files section under package.json

Creating new generators set

Go to the root folder then use the generator-generator generator to create a new generator set then add the name of generator to the package.json

Moving an existing project to a project

Create a Folder inside the generator set to add (if it's a javascript project like react create a folder under javascript).
Create a templates folder with the contents of the project inside of it.
Copy the index.js of another project, it should mostly copy the contents of the templates folder however if you need more logic this is the file to edit.

After copying the repo go to the generator folder and use the command

yarn link

You should then be able to see that generator when you use

yo --generators