NH,

To secure newel posts to a finished floor or tread, we use "rail bolts" which are usually 3/8" or 1/2" fasteners with one half of the length having lag screw threads and the other half of the length having machine threads to take a nut.

We use either 2 or 4 of them depending on the size of the newel. You have to bore into the side of the newel to access the top of the rail bolt and put the nut on. Sometimes you have to angle the rail bolts as you install them to make sure you get into a joist or other framing member. You can then bend the part that sticks out of the floor to be plumb.

I haven't found any other way to get newels installed on a finished floor stiff enough.

Gabel