You cant. Those are your options. They are virtual floor areas and do not have to match up directly with your actual floors. At least not yet. We are trying to get Cisco to go to a 3d map rendering engine to assist in viewing interfloor bleed through issues. Then each floor would have to be completed in its entirety and stacked in the appropriate position on top of its lower floor. As it is now you could in theory split floor 2 in half to accomodate a large number of APs. Each half would have a floor identifier such as 1 or 2.
it didn't answer the question on how to add floor 0 or ground floor.
as it can show floor 1 to 10, it should be on DB, and it should be easy to add to the drop down 0 or G in adittion to the rest of the options.
3D option is nice, but will not be the solution for that.
In your design, floor 1 in WCS IS your ground floor.
Floor 1 in your building will be floor 2 in WCS.
Floor 2 in your building will be floor 3 in WCS
Floor 3 in your building will be floor 4 in WCS.
So on and so forth. n+1
Don't hold your breath waiting on Cisco to fix the mapping issues in WCS. I have been begging for an overhaul for years. 3d and realtime reporting are top on the list.