I'll say it again: You can modify it anyway you like, the only thing you can't do is post the code or your modified code under a different name or distribute it any other way than the main distribution channel for the project.
Uhmm, but that means we can't fork your project on github and how is we then going to send you pull requests?
Join the dev team. Developers interested in contributing to the project will get pushing rights to the original repository.
If you are interested send me your github username and I'll add you to the project.