I'm not really sure what you want to accomplish here. Are there any problems with using a Radio Button?
If there is one, what I can suggest to you is to:
* Update the custom field and add the choice of "NONE"
* Update your workflow and add a condition/validator that this field should be updated during this transition
P.S. If you are worried of a pre-selected value (like it's ticked on YES all the time), maybe it's not the tool that should be adjusted all the time :)
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