Greetings! The PhET simulations can be adapted and modified. Some notes:
* Please be aware your changes must be compatible with the licensing rules (GPL)
* Please be aware the simulations are non-trivial and this work is time-consuming.
* You may also wish to look through the issues at https://github.com/phetsims/build-an-atom/issues
and see if there are already proposals for what you envision. Alternatively, you may wish to create a new issue describing the design of the proposal so the team is aware what you are envisioning and why it would be valuable.
Best Regards and Good Luck,