No, these are fixed and you can not add one there. When you change the layout of the table style by setting all kind of properties this will change to custom. You can create a theme from this table (or any other object) by clicking on the theme maker button in the layout tab of the object properties. After creating and selecting which properties should be included, you can save the theme and apply it to e.g. a sheet or the document
about theme you can read To Theme or not to Theme
Sorry, but where is Preference --> Style --> Current style drop down? Perhaps Properties >> Style >> Current Style
I don't know how can you add element to these list but perhaps there is a HKey in Windows register that give you this possibility.
Luca Jonathan Panetta