IIRC, the plug-in will use the Poser setting.
Disable it in Poser and it will be disabled after the conversion to the plug-in as well.
I often use something like Snarly's "Scene Fixer" script, to change multiple things at once. One setting is the "Set smoothing" option in there.
The Octane smooth setting is per material, whereas the Poser smooth setting is per object, and the Smooth setting is Poser is NOT obtainable from Poser - so unfortunately the plugin cannot set the smooth pin for a prop/figures material from the Poser setting. Instead, the Poser smooth setting effects the actual geometry that the plugin gets.
Is it possible to change default setting for octane material to have smooth disabled?
Example: I import a building where most objects are flat (walls, etc) it would be nice if i could change defult smooth enable to disable.

Or is there a way to change multiple material at once?
Since the majority of users user Poser for human figure rendering, and human materials need smoothing ON, the default setting is ON. I may however be able to add a configuration option where you can set the smoothing setting that will be set when next converting materials.
Paul