robin, it's possible but it's not easy.
You need to first use 'addLine' to place 10x10 blocks, and then use checkBlock/removeBlock/addBlock to check the entire map and replace the 10x10s with 20x20s. And that would require a 60x60 map, because with a 50x50 then you'd also need 10x20 or 20x10 blocks and more scripting.