Thanks for the info, those are all good ideas.
Crono, I never heard of making a map in a modeller. I just make the
whole map in Blender and then port it in? How would I texture it? I've
never played the Ghost Recon map you're talking about
You would texture it once it's all in hammer.
Anyway, it's a bad idea, and a pain in the ass! I'd only suggest it if you want to make incredibly complex structures.
Otherwise, if you have the blueprints of some layout you want to make, it has measurements. You can use those in HL2 hammer, everything is in inches though. I know you can change it to feet and inches, and that changes the measurement at the bottom of the screen, almost on the task bar. That'd be the easiest way to recreate something.
But, to answer your questions it IS possible. It'll suck though. Be hard, and yield very poor results.
The reason I mentioned GR, is because A) It's old and B) all the maps were made with 3DS Max. There is no level editor for the game, I think there's one for entities so you can put in that sort of stuff. But texturing and everything else is done in the modeling program.
Because the entire map is a model, it is very efficient. There are NO world calculations so the game runs like a dream in an incredibly large expansive environment.
The same would happen in HL2 ... it's just really hard and not really worth it. That was my point though. It'd run faster. But those are compiled models.
The question you're asking is "How to move an un-compiled model to hammer". Which, I said is possible, but I don't suggest.
Anyway, this is long winded. If you don't want to read it all:
You can transfer an un compiled model to hammer through various programs (I believe those modeling programs have a to MAP type of option)
However. That will allow the geometry you made in the modeling program to be world brushes in hammer ... it just isn't a good idea.
If you want to model the entire building, however, that'd be a very interesting idea! I would imagine that'd run pretty damned smooth. Just be aware HL2 requires world brushes for vis blocking.
Blame it on Microsoft, God does.