Translation:I will add that I did not see him.
' will add' doesn't mean will toevoegen in this context. It should be 'zal toegeven' which means 'to admit'. If it is suppose to mean that on top of something else you add that you didn't sees him, then the translation is also wrong, cause in dutch you don't use this construction. It could then be something like: Bovendien geef ik toe dat ik hem niet zag.