Use Map Demerger. It splits M1 maps into a Forge-friendly format. Things like doors and platforms values are preserved. Lighting is preserved also, but only on the floor and ceiling, not walls; also some lights that are flagged are primarily active are not. Monsters that appear only on M1, like the minion, are converted to something else, etc. A real time-saver. You still will have to extract the terminal data from the Marathon App. with ResEdit and change some polygon types.
BTW, I tried to contact deRuyter's author and the mail bounced back. Just do a decent conversion and I'm sure nobody is going to sue you.
