I've been playing around with terrains and this works best. Here's the picture . In the center is the original terrain 524288 polys and a bryce cube 6 polys. I set the terrain negative and the cube positive. I copied the two. I set the static and selected wireframe resolutions to 128. I found out that if you change the wireframe resolution after you group it doesn't change the final size resolution. After this I grouped and combined the two. They produced a combined mesh 528473 polys. I exported this by going to the top bar in bryce and using the export object command under the file menu. I loaded the exported object into Bryce this came in as a group with only 4179 polys. It's possible to ungroup and apply a different texture to the cube or the hole.

I pasted the original terrain and cube again. and moved them to the left
I went back into the wireframe resolution and set it to 8.
I grouped and combined, exported, and imported again.
Notice the new object is only 62 polys but the hole is rounded off and much of the detail is gone.