This would be pretty strightforward using a design-time script. I don't know if you're all aware that GameDev can use scripting not only at runtime but also at design time, like in the map editor. The sample game that comes with GameDev (with the wizards in the dungeon) has an editing script that gets connected if you choose the "Edit Dungeon Project" shortcut from GameDev's program menu (in the start menu). It handles automatically filling in the torches and tops of walls. You just draw the walls on the main layer, and then press "F" to fill the currently visible area of the screen with the wall tops that match the walls you've drawn. Or press "A" to full the entire map with the wall tops. You could do something similar to copy tiles from another layer in another map. Look at the DngnEdit.vbs file to see how this was done.
I believe GameDev 1.4.6 was delivered with a GameDev.vbs file, which contains default script that will connect whenever you use the map editor with any project. You could add some modifications to this file if that's working. Or you could copy MapEdit.vbs, a template for designing your own map editing script.
If you need to get the map into the project, you might be able to do it by just creating a similar blank map in your project, then exit GameDev, delete the file containing the blank map and copy your other map file in it's place. Next time you load the project it will hopefully pick up the map file you copied. You should probably make sure you have all your project files backed up before you try this.