I come from Hammer too.
You can not use it to make unity levels.
At most you can create a level, export it to obj or xsi (been a long time so I forget)
Load the exported file into softimage xsi, delete all backfaces and merge the small objects into groups.
Then unwrap/texture the models and export to fbx into Unity.
But this is not an effective workflow and I only used it to convert the one level I had when I switched from source to Unity3D.
On the asset store there are now some tools that enable you to create objects inside unity and allow you to build a level inside unity. I have no idea if this is recommended or not performance wise tough :)
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