Nice idea - won't work. Why, you ask?
When you create a map in HL (or HL2, as the case may be) - you create a file that has all of the information for building the map. All of the walls, textures, entities (such as the piano in Ava, flower pots, fire, etc).
Then the person who built the map "renders" it. Changes it from the descriptive, editable file - where walls are lines on the screen, with the texture information (and lighting information). If you've seen how Pixar builds cartoons - much the same thing.
This rendered file is what is shipped with DOD. The "source" file doesn't come with the game.
In HL(1) days, like Counter Strike, and DOD Classic, you could take one of the old BSP files and (mostly) decode it into a map file. The problem there, is that the file would have no textures (think, white walls) and you'd have to re-do a lot of the things inside of the map. It was a lot of work.
To do what you're talking about in Donner, you're talking about re-doing the entire map - from scratch. Probably .. I don't know .. 500 or more hours of work ..
Adding or removing entities on an existing map is not horrible to do. You can use server side addons to do it (and in the old HL1 mods like CS and DOD classic, you could actually put the entities into the compiled BSP)...