In recent versions of Scoop, the vote form is a block, so you can change it as you feel necessary, such as by removing the Abstain vote or changing its label to "I don't care."
It would take some code changes to add a checkbox labeled "I don't care," but how does that help anything? It just makes the interface a little more complex while maintaing what amounts to a 0 vote. Having a drop down and a checkbox implies that they are for two separate things, when in fact checking the box causes the drop-down to be ignored.
Maybe what you want is for the drop down to have no value by default, so that a user has to choose something, instead of being able to just click on "Vote."