Provide a preset response type that would include these options for members to select from:
- Other: [user adds custom input here]
Up voting assuming you mean to have the option of pre-filled responses as opposed to always requiring it. In some cases, I can see where defining the domain of answers is helpful, not just Yes/No. To Mike's point, polls (surveys) can be misleading if the right answers are not available or if the question is off. so you'd want to be careful how the questions/answers are put together.
Really a bad idea, unless you want to tilt the result, then a good idea