Hi,
You can use any model in your scene as
Custom Object under
Geometry tab. So, if you
Merge (or link as
XRef) any model into your scene and then create Forest object you can pick it as
Custom Object.
Also, in the "Pro" version you are able to add any model(s) into your
User's Libraries, for easy use with
Library Browser, please check this video tutorial about
Creating New Presets. In our
on-line reference guide you can find detailed information about
Library Edition workflow.
Hope you'll find it useful.
Kind regards,