You can't have different materials on the top of a mesh. That's what ArchiTerra is for. (Yes, I saw your other topic.)
Do you mean you need an "island" of one material surrounded by another? If so:
1. Copy the mesh in place, so you have two identical, coincident meshes.
1a. At this point it's a good idea to change the pens and material of one of the meshes so you can tell them apart going forward.
2. Split the island mesh along one of the island's edges, and delete the outside piece.
2a., 2b., etc. Repeat for the other edges.
3. If the shape has interior corners, you need to boolean-subtract those areas. Curves are tough; I don't think you can split a polygon with an arc. You'll need to approximate them.
4. Once you have the island mesh trimmed how you want it, use Solid Element Operations to 'subtract upwards' the island from the big mesh.
As you split the mesh, AC adds nodes on the edge to keep the edge polygons in place. This means you can't move nodes around afterwards, or the two meshes will get out of sync, and you'll never fix it manually. Make sure your big mesh is really done before starting.
If you have to start over, just delete the island mesh. Since you used SEOs to cut the whole, the big mesh is back to its old self instantly.
HTH,