In Importacular, you can work with Data Transformations to add a Yes or a No value to Yes/No fields. These Data Transformations function similar to a Data Transformation on a field that is Table based in that you have to do a Replace Type of Complete (as the field must be either Yes or No).
Below, you can see an example of a Data Transformation working with the Do Not Contact field on an email address. The Data Source contains an email opt out date and this transformation is using that date in order to indicate that this email address should not receive email.
When we open the Field Settings, here are the Data Transformations:
Here, we have set this field so that if there is no opt out date, the box will be left unchecked (the first row).
However, if the box has any value in it (we've used the Catch All at the end of the row), the box will be checked.
When working with the Yes/No field, you could also use other Match Types and assign those values to either a yes (a checked box) or a no (leaving the box empty).
To learn more about Yes/No field options, please see this knowledgebase article.