At (nearly) any part of the software you can insert a text-field via HTML, even if there's no particular feature that would allow you to do so otherwise.
First of all you are going to need a 911 question in order to create the variables. Please make sure to use the correct variable-types, which is especially important when you want your participants to put in text elements.
Next step is to insert the HTML-code at the intended place:
<input type="text" id="v_x" name="v_x" value="#v_x#" style="width: 200px" maxlength="47" />
Of course v_x has to be replaced with the name of the variable you created in the first step.
style="width: 200px" controls the displayed width of the field and maxlength="47" controls how many characters the field will accept.
Additionally it might be necessary to pre-define the variable's value with either an empty value or 0 by using a recoding trigger.
Further information regarding input-field in general can be found at: