Vegetation data does not translate well to Unity and UDK, there's no easy way to move it unless they are using GROME's native Graphite engine (such as the Ogre3D Graphite rendering module).
The most practical method of transferring vegetation coverage to Unity and UDK is through the use of coverage masks. GROME uses masks to populate vegetation within zones and these are saved as image files by several of the built-in exporters.
These masks can (with some effort) be imported into Unity and UDK but this presents practical editing issues should we need to perform any fine-tuning to the game map. Once a bulk transfer of vegetation data has been completed, any edits later on become more difficult to integrate, especially if it's required to reimport everything, potentially losing many hours of work. For this reason I usually recommend using Unity or UDKs built-in vegetation tools from this point onward but there are no hard-and-fast rules.
