Look at the pinout. All of the five pins in the DIN5 part of the connector are already taken. These are the first five pins when counting the pin numbers. As a sidenote, this is why the DIN pin numbering seems "weird". Pins 1-3 are the pins present in DIN3, which is three pins placed at 90 degree angles. Then pins 4 and 5 between those, with 45 degree spacing. And finally DIN8 which adds three more pins, while keeping the existing pin numbering the same.
Anyway, even if there's no free pin you can use immediately, you could cut the PCB trace going to the luminance connection and route a wire around the board to connect to that solder point. Assuming you use composite for video and not component. However, that would require a custom cable that would only work with your C64. In the end, it might not be worth it. If you're not afraid of drilling in the case, you could just as well add RCA or 1/4" jacks which are more standard and might serve you better in the future.