Hi

If you mean adding in custom environments, then no, we don't plan on doing so.
If you mean an SDK for creating tracks in races other than in the Liftoff Track Editor, then that's already possible. Liftoff uses XML for its track and race files. So you can create them easily in other software if you want or like.
Note: we're working on a new track editor, and some of its formatting will change, so if you plan on doing that, perhaps its best to wait for that to be released.