Is it possible to convert an "edited" map in GMod (i.e. one on which I have placed things) into a "new" map (i.e. one that will have those things on it by default)?

Here's what I mean. On gm_mc_flatgrass, I am in the process of building a castle from stone brick blocks, by using the Minecraft SWEP. The problem which I have noticed already is that with so many blocks, a lot of lag is present. My eventual hope is to be able to play team deathmatch using the castle as the main environment.

What this would necessitate is the following:

1) Turning the castle from a bunch of separate blocks into one unit. There's no doors or anything, just stone. so no moving parts.
2) Making the castle a part of the map, like the buildings in gm_construct.
3) Exporting/converting the result as a new and separate map.

Is it possible to do this? Google has been little help, since it seems to get the words "import" and "export" mixed up when displaying results.