Yes! To add a fill-in option to your multiple choice question, scroll down to "Add other" and toggle it on. This will allow respondents to write in their own answer choice.
We suggest prompting respondents to write in their answer by editing the label to read "Other (please specify)" as seen below.
Similar to the "None of the above" option, including an "Add other" option will automatically anchor at the bottom of all other answers choices, even when you randomize.