Put that turn file into folder in your DomII main folder, where your other game folders are located. Be sure that this folder is not one of the following: doc, mods, maps, newlords, rawsound. Start DomII, choose the game (folder's name). You still need the password, of course.
Play by E-mail games are that simple, and as a bonus you have the turns storaged somewhere they won't be removed. I still have my second Dom:PPP PBEM game in my E-mail account (50 megs)...
Being able to extract battles in DomIII would be nice, but I don't expect to see it.