I have the knowledge on how to make Marathon maps in Source engine.
There are no import tools.
Only two maps currently.
Hats Off to Eight Nineteen in Garry's Mod.
https://youtu.be/0aq3sSf62Gs
The other map I'm copying is Pfhactory N’Utopia.
Make Marathon maps in Source engine
Looking nice! Any way to import 3D models into Source? I recall there were some Marathon 3D map exporters out there. Might help you out.
I never found a way to import .obj maps to the Hammer Source map editor.
Have you seen the Half-Life 1 mod Marathon Rampancy?
https://youtu.be/TIUIsJ_ER-E
I think the maps were imported, but I don't know how and I never found a map exporter like it.
The Pfhactory N’Utopia map was the most complicated map to build.
I'm adding textures to it now and I plan to deathmatch on it.