I'm pretty sure there's a program to convert compiled source maps back into map or vmf files. You could the port it to Blockland with a little difficulty.
It would be easier just to map it from scratch. I mean people have built from scratch out of blocks, so you definitely should be able to map it. It should even be possible to write a blockland build to interior converter considering bricks are for the most part convex but it would be pretty inefficient.