I don't know exactly what do you mean with converting object to rooms, but I tell you how I do it.
1. make empty room with this tool
https://www.klubexile.com/files/file/2544-vx-room-creator-hook-5/
The room is ready but nothing is there, so you have to add your objects to lvl_definition. (F4 hook menu in game and add objects.)
2. create a room in Blender (build walls, add objectes etc...)
3. export parts of the room to objects. (You can't export all room into one object because h5 converter will freeze (too much data))
So you can export for example one chair and place it few times in your room in the game or group of chairs and place it one time. Up to you.
4. Convert obj to h5 object using h5 converter (you can download it from @pervokpetr patreon page )
5. Final tunning (add lights, positions, etc.)