You should do a fork of main SolveSpace repo, then you need to develop features you need, after this you can do a pull request of your feature to the main repo. But before this, it is better to create issue inside main repo at GitHub, where you can describe a problem or a new feature you wanted to implement. Members of SolveSpace project can see this and will help you to choose better way to do it or can point at source code where you should search for problems.