Vegetation populate tool on imported terrain mesh.

https://preview.redd.it/0weov5b7nbnf1.png?width=802&format=png&auto=webp&s=d2a4c531dfe414ced5507fa0c200faed8f270fa4

Hi!, so I imported a terrain from ArchiCad into Twinmotion. The darker green is the ArchiCad terrain to mach the real site, and the light green is the twinmotion terrain I sculpted to surround it. I am now trying to paint vegetation on it with the populate tool. However, when I pass the brush over the terrain it burries the trees into the ground (under the Archicad terrain) because the brush only works on the twinmotion terrain. Sorry if this is a horrible explanation. But does anyone know if I can paint over the ArchiCad terrain somehow? or if I can convert the ArchiCad terrain into twinmotion terrain. Please help it is urgent.