There is a way to do this, but it's a bit fiddly. You need to download and install the Free Open Source Meshlab application as a conduit - http://www.meshlab.net/
You can take your terrain from SketchUp and export it as collada - .dae format.
Open Meshlab and import the collada file. The big drawback with this, is that the terrain when loaded into Meshlab is not real world scale you you need to scale it in Meshlab. There are instructions for this here: https://www.google.com/search?q=meshlab ... &ie=UTF-8
Then you can export to an xyz file (use the without normals option), and use Archicad's 'Place Mesh from surveyor's data' option.
This gives you a true scaled SketchUp mesh in Archicad. The next thing you need to do is scale the texture to fit the mesh. The texture is imported, but there is nothing in the xyz file to map it to the mesh, so you need to do it manually.
Unfortunately Archicad will not allow you to scale a mesh in 3 dimensions. You can scale in x and y, but not z, which flattens your terrain. This means that scaling in Meshlab is your only option.
If only Graphisoft would allow us to import 3d terrain from other file formats and create a mesh instead of a morph. Or at least allow us to convert a morph to a mesh...
Here's the result.