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Are you referring to the "class" field?
http://s11.postimg.org/5oa1da2n7/insert_table.png
I suggested a solution in a different thread: http://world.episerver.com/Modules/Forum/Pages/Thread.aspx?id=76138. There is no indication if it actually solved the problem, but you could give it a spin
Is there a way for adding my own styles into the dropdown in the Insert table editor?