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Pretty much the same as you do for objects.
In S3OC, go Cloning -> Wall/floor patterns, pick a floor to start with, and hit "Clone or fix...", make sure "Rename/renumber" is ticked, and OK. Then open your new file in S3PE, export the _IMG resources, edit them however you like and then reimport them.
Or, if you prefer, go to TSRW's New Project window, pick a floor (I think they're in the Build objects section), export the textures, edit them, and reimport them.
Do you need help with editing the mask, normalmap etc.?
What I lack in decorum, I make up for with an absence of tact.