There has been some work on a new tool that could help.
I figure the best bet for such a converter is to export the DF maps layer by layer and set the converter to scan the resulting bitmaps for images for specific tile types. Of course many things could cause shenanigans like vermin in the air appearing as ground tiles and other such things. Or clear glass windows looking like sky tiles. Or magma looking like red sand.
Heck, one could just set options in said program to have all tiles turn into a default(So unless one wants a lava lake map they can set all tiles of that type to turn to sand, including any magma pipes) to get some working concept out there. It will suck having to repair these problems but it will give time to work on a system before somebody on the Bay 12 Games forums thinks of doing this and helps out.