We have plans to release Forest Pack (& RailClone) with 3ds Max 2018 support soon after V-Ray for Max 2018 becomes available. We'll keep our users posted about this topic in the announcements thread. I should add, that because Forest Pack has many dependencies on the mental ray libraries (which are now not included in the 3ds Max 2018), it needs to be treated very carefully. Thanks for understanding.
I hope that helps, please let me know if you have any further questions.