Can I freeze undercooked (pink) chicken to finish cooking later on?
It is entirely fine! The only thing I would worry about is when you are re-cooking them. Let them come down to room temperature before you throw them on the heat, otherwise, you'll get overcooked/burnt edges and a barely cooked centre. I would just be wary about keeping them in the freezer for over two weeks, not for health and safety, but just for taste and culinary standards. 

For when you take them out for your meal; enjoy!