I was just wondering if it is possible to use a pre-mapped space as a background for hololens and to add some holograms in it.
I also would like to know if it is possible to use the spatial mapping only on the object of my gaze.
You can do this with Spatial Understanding. The capability to save and load the spatial understanding mesh has not been built into the holotoolkit yet. This will eventually get integrated into the HoloToolkit I'm guessing, but for now you will have to do it yourself. There is some great example code of how to do this in the issue requesting this feature: