i know its old, but i ran 4 years later into the same issue and maybe other people will do.
create level + sublevel + landscape.
paint foliage.
paint SELECT foliage now you want to move to sublevel.
go to Levels Window, right click on your sub-level
and click “move selected foliage to sub-level”.
and a nice trick, if you just want to make a copy of you foliage and not move it. just move it from lets say sublevel a to b and do not save sub level a.
if you load next time your master level, both sub levels have the foliage inside.
i hope it works for you 