It arrives when Season 3 does for the MMO, which also lets you replay missions and use Borg tech on your ship. Cryptic moderates all Foundry creations.
Officially the Foundry tools are still in their beta but it will become available nonetheless as the developers are confident it can perform. You won't be able to import your own assets or anything as it is not a mod toolkit, it's just a mission scripter.
Star Trek Online's Season 3 update also gives Sector Space a good remodelling to make it appear more Star Trek-like, i.e. more interesting for the community. STO may adopt a free-to-play model if Cryptic's Champions Online does well with the changeover.