How add point to boundary of floor?
Not easy.
Discussed in 2008:
http://thebuildingcoder.typepad.com/blog/2008/11/editing-a-floor-profile.html
You may be able to use tricks to access the floor sketch curves:
http://thebuildingcoder.typepad.com/blog/2011/11/undocumented-elementid-relationships.html
Modifying them is probably harder and maybe not possible.
However, as you see, you can read the existing curves and create a new floor based on those plus your additional boundary point.
Cheers,
Jeremy
