I found a question I also have can you add another option to an option group without rebuilding the group
has these answers
1) Select the frame for the option group and size it to allow for the new option button.
2) Select one of the current option buttons within the frame
3) <ctrl><c> to copy
4) immediately, <ctrl><v> to paste
4a) this inserts an EXACT copy of the option button you copied so we're goint to have to change some things!
5) Show the properties for the new option button
6) All tab: Change the name and the caption (if used)
7) Data tab: Change the [Option Value], check the other other option buttons in the group. If the [Option Values] are the same then all buttons with this value will toggle at the same time (I've done this by accident (^_^)).
8) If you had any event codes (ie OnClick) for the button you copied, you may need to modify it depending on the procedure.
But something must be missing as the added button does not toggle with the rest (it is on/off) and the properties are different e.g. new one has "triple state" and more options under Data tab. Also the group ones show no control source.