It's funny you say it's hl2dm map. I d/l'd it and opened it. One of the lines speak of wad files?
Couldn't get it to work in anything, sorry. I tried .map/.rmf formats. Essentially any .vmf should be able to be opened in .rmf as well (valve haven't changed the way the files work afaik) with missing props etc, but solids intact.
EDIT: seems like some did better than me. gj ifo!