Yes you can select the curve and make it a planer.

This will produce a new object.

Then you will have two peices to combine together. Under Mesh rollout

Select the vertices of the new object around the edge and under the Edit Mesh rollout use the merge tool.


I suggest you make sure the merge tool is set to .001.

Also you should check the face normal of your top to be sure it is facing the right way.

After that you need to be sure the top is properly typologized. Meaning quads if you can.

Hope this helps